Horizontally/Vertically Straightening a Shape in Illustrator

Question

Let's say I have a shape that I rotated at an awkward angle of let's say 34°. I close the document, return a few hours later, and want to straighten the it back to its original 90° or 0° angle. The problem is I don't remember the angle I originally rotated it at. Shift + Rotating nor resetting its bounding box does the trick. Any shortcuts/options I'm overlooking here? For example how would I rotate this quadrilateral so that it sits horizontally straight and looks like a box.

Answer 1

Using smart-guides, it's pretty easy to make a new guide and align your edges to them. Set the center of rotation to intersect the lowest desired edge and a guide, then rotate until both are collateral.

Answer 2

You can also get a free plugin from Astute Graphics for just this very situation - http://www.astutegraphics.com/software/subscribe/. As simple as selecting the points on the edge you want to be horizontal, holding shift to constrain the angle and dragging.