Declaring a variable of a variable?

I'm working on using input from the mouse to move a sprite in DirectX.

I'm trying to use this example from the professor, But you see my commented error:

// Mouse
float mouseSensitivity = 2000.0f;
spriteObj[5].position.x += (mouseState.lX * mouseSensitivity) * dt;//Error  1   error C2228: left of '.position' must have class/struct/union   //Error 2   error C2228: left of '.x' must have class/struct/union  

I am trying to initialize the variables position and x of "spriteObj[5].position.x", But How to I declare x as a member of position as a member of spriteObj[5]?