Height of a mc at a certain _x position

Please take a look at the following:

Alien

What I want to do is make the alien walk over the red line.
So I guess,… I need the height of that redline movieClip at a certain x position.

I need some code like this in the alien_mc ( but than working):

ActionScript:
onClipEvent (enterFrame) {
this._y = “height of the redline_mc at the same _x position”;
}

Is something like that possible? How can it be done?

Thanx in advance.