AS3 Random Spawning help

Hey everyone! :smiley:

I have enemies spawning randomly in my game and the code below decides where they spawn from.

function onLoad()
    _x = 700;
    _y = Math.random()*300;
    speed = Math.random()*5 + 5;

It all works fine with my other code but my real question is if there is a way to make my enemies spawn randomly from a movieclip, not just a fixed point on the screen, with the code deciding the random spawn position somewhere along the width of the movieclip.?

Any help would be greatly appreciated! <3