When key pressed rotate until lifted if key up smoothly revert to original position and if rotate too far die

Question

I am trying to have a smooth rotation animation on this hover-car to make it seem more realistic.

I am trying to code it so that if the key a or d is pressed it will rotate to the left and right accordingly. If it rotates too far then it dies and the scene is reloaded. When the keys a or d are lifted then it will smoothly revert back to its original position. I want it to rotate on the x axis of its origin. If you see in the image the scenes axis is z and the axis of origin that points in the direction of that from the vehicle is its x.

Here is the scene with the axis's of the vehicle:

        public int speed = 100;
        public float maxrotation = 30f; //To Set Max Rotation

        void Start()
        {
            
        }

        // Update is called once per frame
        void Update()
        {
            if (transform.rotation.eulerAngles.x > 0)
            {
               
                transform.rotation.eulerAngles.x - 1; //there is an error on this line saying "Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement"
                
            }
            if (transform.rotation.eulerAngles.x < 0)
            {
               
                transform.rotation.eulerAngles.x + 1; //there is an error on this line saying "Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement"
                
            }
            if (Input.GetKey(KeyCode.A))
            {
                //Set a rate at which we should turn
                float turnSpeed = speed * Time.deltaTime;
                //Connect turning rate to horizonal motion for smooth transition
                float rotate = Input.GetAxis("Horizontal") * turnSpeed;
                //            //Get current rotation
                //            float currentRotation = gameObject.transform.rotation.z;
                //            //Add current rotation to rotation rate to get new rotation
                //            Quaternion rotation = Quaternion.Euler (0, 0, currentRotation + rotate);
                //            //Move object to new rotation
                //            gameObject.transform.rotation = rotation;
                gameObject.transform.Rotate(Vector3.left * rotate);
            }
            if (Input.GetKey(KeyCode.D))
            {
                //Set a rate at which we should turn
                float turnSpeed = speed * Time.deltaTime;
                //Connect turning rate to horizonal motion for smooth transition
                float rotate = Input.GetAxis("Horizontal") * turnSpeed;
                //            //Get current rotation
                //            float currentRotation = gameObject.transform.rotation.z;
                //            //Add current rotation to rotation rate to get new rotation
                //            Quaternion rotation = Quaternion.Euler (0, 0, currentRotation + rotate);
                //            //Move object to new rotation
                //            gameObject.transform.rotation = rotation;
                gameObject.transform.Rotate(Vector3.left * rotate);
                
            }
            if (transform.eulerAngles.x > maxrotation)
            {
                Application.LoadLevel(Application.loadedLevel); //kills player and reloads scene
                scorecript.scoreValue = 0; // sets score to zero
            }
            if (transform.eulerAngles.x < maxrotation - (maxrotation * 2))
            {
                Application.LoadLevel(Application.loadedLevel); //kills player and reloads scene
                scorecript.scoreValue = 0; //sets score to zero
            }

        }

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Answer 1

// Cache the rotation we had at start-up.
Quaternion originalRotation;

void Start()
{
    originalRotation = transform.rotation;
}


void Update() {
    float axis = Input.GetAxis("Horizontal");

    // If we're not pressing left or right, rotate back toward neutral.    
    var targetRotation = originalRotation;

    // If we are pressing left/right, our target should be somewhere to the left/right.
    if(!Mathf.Approximately(axis, 0)) {
        var twist = Quaternion.Euler(0, 0, 90 * Mathf.Sign(axis));
        targetRotation = transform.rotation * twist;
    }

    // Smoothly rotate toward the current target, at turnSpeed degrees per second.
    transform.rotation = Quaternion.RotateTowards(
                             transform.rotation, 
                             targetRotation, 
                             turnSpeed * Time.deltaTime
                         );
}

This code uses a linear rate of change, but you could also add damping/easing in and out if you like.