Calculating the position of an object with regards to current position using OpenGL like matrices

Question

i have a 1st person camera that collides with walls, i also have a small sphere in front of my camera denoted by the camera position plus the distance ahead. I cannot get the postion of the sphere but i have the position of my camera. e.g.

i need to find the position of the point or at the very least find away of calculating the position using the camera positions. code:

    static Float P_z = 0;
    P_z = -15;
    PushMatrix();
    LoadMatrix(&Inv);
    Material(SCEGU_AMBIENT, 0x00000066); 
    TranslateXYZ(0,0,P_z);
    ScaleXYZ(0.1f,0.1f,0.1f);
    pointer.Render();
    PopMatrix();

where Inv is the camera positions (Inv.w.x,Inv.w.z), pointer is the sphere.