Next/previous in sliding menu

Hi there, I’ve done this somewhat dated but great tutorial for a sliding menu:

http://www.kirupa.com/developer/flash5/slidingmenu.htm

You can basically place 1 large banner under a mask which shows only a small part of the banner and use buttons to navigate the mask to certain, pre-determined area of the banner.

The code for a button is like this:
home_btn.onPress = function () {
_root.xnew = _root.mask2._x+(1-1)*1000;
}

‘home_btn’ in this instance being linked to the first 1000px of the banner (the width of the mask). Now I’d like to add a ‘next’ & ‘previous’ button, but these are dependend on the location you’re at, so this code doesn’t suffice, because it generates a fixed location, no matter where you are. How can I alter this code, so that you either go 1000px forward, or 1000px backwards from the place on the banner you’re at?

I tried something like this:

prev_btn.onPress = function () {
_root.xnew = _root.mask2._x-1000;
}

next_btn.onPress = function () {
_root.xnew = _root.mask2._x+1000;
}

But that didn’t it work (“duh”). So what should it be? Any help much appreciated!