Sliding MC position relative to contents

I have a movie clip that I have given the a “sliding” variable to move it across the X axis.
So far I have been using a variable of

movieClip.newX = -250; 

, but what i need to do is have the movieclip slide to a position which is relative to an object inside of it.

As i need the slider to move to a position that centers the object which the slider contains on centered on the screen - i have tried:

movieClip.newX = movieClip.ContentsObject.Stage.width/2;

but with no response. I realise that code is incorrect, but need something that will work in that way. I presume i need some sort of listener to follow the _x position of the specific movie clip.

Does that make sense? does anyone know?
cheers,