Please help! Simple Slider question

Hello
I am very much a new bee to Flash. Have been working with it for less than a week. This is what I am trying to do. Any help would be greatly appreciated.
In the library, I have a movie clip - DraggableButton that has OnPress with StartDrag with parameters and OnRelase with StopDrag. I have a slider with this Draggable button and a bar to slide the button on. I also have two movie clips Clip1 and Clip2.
In my main movie I have two instances MC1 and MC2 of Clip1 an Clip2. I have an instance of the Slider.
I want to set the _alpha property of MC1 and MC2 depending on the location of the button on the slider.
I think I need to save the location (_x) of the button on the slider in a variable in the OnRelease even of DraggableButton and use this variable in the SetProperty method.
I need to know in which event and which object’s event should I use the SetProperty method. Or if what I am thinking is not correct what do I need to do?
I know though this is hard for a newbee like me, it is not very hard for you Gurus. Please help!
Thank you very much.