Although you can use this type of equasions they are quite unhandy. These are derived from intigration where a fragment of time known as delta time or even dt limits to zero. They are most precise but usually unfit for physics engine due to the complexity of finding the time at which the object or projectile will collide. The complexity grows with more dynamic objects and once those dynamic object may appear at any time the system simply becomes unmaintainable. In other words I highly recommend you not to use them...
So what it is best to do is create a fixed delta time which is most commenly the duration of your frame or 1/FPS seconds.
The eqasions are then as easy as:
position = position + speed * dt
speed = speed + force * dt
Then on each frame you would call for each projectile to move in which you would compute both, the new speed and the new position. After you have moved the projectiles you should check for crashes and handle them if any.
Note that all of those parameters I used are vectors except the
dt. The gravity in your case would be a constant force vector of (0,G).
To add a bit on your specific case: if the user input would be an
angle at which the projectile would be launched and its absolute speed
aSpeed then all you needed to do in the begining is to determine the
speedX = aSpeed * cos(angle)
speedY = aSpeed * sin(angle)
Then if for instance your gravity acceleration is 10 (m/(s*s)) and your FPS is 10 you would in each frame call:
speedY = speedY - 10 * 1/10
positionX = positionX + speedX * 1/10
positionY = positionY + speedY * 1/10