Of course I'll scroll one dimension at a time! But since my first project is infinite horizontal shooter, it isn't that important. Just wanted to make sure my equations are right.

Also:
Lets think it through:
Object is in position 100, 100
Camera is in position 90, 100
So we expect that the object will be shown slightly right from camera location. With above formula it becomes
100-90, 100-100 = 10, 0, which is slightly right
No, (10,0) is slightly LEFT. (0,0) in games is top left corner.