I've been using Brightstorm recently to prepare for the next lesson, it's great but occasionally you run into some questions and I don't believe they get answered any time soon.

I'm watching this video in particular and I am stuck with the second solution for

Solving Trigonometric Equations (Video)

(Picture)

If (pi) radians is equal to 180 degrees and all three angles of all three triangles is equal to 180 degrees, why isn't the supplement angle ?