Left angle represents the left line measured anticlockwise from the top. Right angle is the right line measured clockwise from the top. Both can be greater than 180°, though that doesn't do anything special.
Either edit the angles in the boxes below, or (much more fun) use the diagram. Click left of the origin and drag to change the left angle. Start on the right to change the right angle. You can move to the other side, but only certain ranges will yield proper results.
Formulae adapted somewhat horribly from here. Couldn’t find equations anywhere else.