Expression: Obj 2 moves right when Obj 1 moves left.???

Hello.

I having trouble trying to come up with an expression that would allow me to move OBJ 2 to the right when OBJ 1 moves to the left.  Obviously when I parent one of the objects to the other and move to either side the both end up moving in the same direction.  Does anyone have an expression for this or perhaps can lead me to a helpful link that would educate me.

Thanks in advance.