Quick math problem

hey all
after two near allnighters my head is litterally turning to mush.

I have am wanting to do a quick calculation based on the mouse position.
here is what I am trying to do.
I have an item the site on the stage

stage.width = 400;
object.width = 900;
object.x needs to be at its most -900 and at its least -225.

and based on the mouse position on the stage object will scroll within these bounds.

I have all the functional done but just need to know the basic equation to find these points.

Merci mille fois