The 180 rule sounds simple: the angles of a triangle always add up to 180 degrees. And it is simple, until they start throwing in x's, and 2x's, and 180-2x's... You get the idea.
Triangle Exterior Angle Theorem
This is one of those things you have to learn "just because". You won't see it again, you may not even see it on the test, yet it will be on your homework and you'll waste time studying it because you think it might be on the test. Oh well, at least you can do these problems with the 180 rule instead!
180 Rule & Exterior Angle Theorem Proofs
Proofs are so awesome. In this video we prove the Triangle Exterior Angle Theorem, which is probably on your homework assignment.
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)