Okay. I guess you're okay with BDE is "2a"? Well, we know angle AED is "a". And this angle is bisected by the vertical line in the picture, so the line AB must be "a/2" from the vertical, either side of the line. Now, if we draw a vertical line down through D we have just shown that DB must be "a/2" from the vertical - as the full angle BDE is "2a", this means the line DE must be "2a"-"a/2" from the vertical - i.e. "3a/2"
Given that:
re(2): Oh wait - I gottit!
