Check out the attached image for the full two-column proof. Feel free to ask any questions you have. The proof hopefully is self-explanatory. The only thing I would like to make a note on is a "linear pair" is where you have two angles that are adjacent (touching each other) and they form a straight angle. Those properties apply to angle 1 and angle 3, which is why they are a linear pair. So that points to them being supplementary (they add to 180 degrees).