rotating edge/face to parallel with axis

Simple question. I need to rotate and edge so that it’s parallel with an axis.

http://i456.photobucket.com/albums/qq284/pellaamarilla/havethiswantthis.png

I need it exact, not eyeballed. Easiest way to do this? Thx

Wait, I’m an idiot. I can just do a snap to with align rotation on to a surface parallel with an axis.