Resizing dilemma

Hi.Im not advanced in AS, but this is an AS question.

I have a content box, it has to be centered. easy.
i have a margin, or line, whose width must match the stage width. easy.

these two things i have been able to do okay with the AS help of the good people in various forums (i will return the favor when i learn how !!).

okay, now i want to have the border/margin to also resize with the screen, like the content box. this is my problem…im trying to have the line stay with the box when it is resized. at the moment all i get are two, equally undesirables:

either the line refuses to reposition and is cut off.
or (as in the example im posting here) it stays exactly “centered” and i can do nothing with it.