Hi Jason. I'll give you a few hints below, but wanted to say a few things about the homework too. For MC III the problems are meant to be challenging, and often require a trick or two to solve the question. Therefore, it's okay to be stuck! We don't expect you to solve all the questions each week, but it is important to try and see what progress you can make. When asking questions here as well, try to explain a bit about what you've tried so far, and why you think you're on the right track or why that method didn't quite work. Sometimes for harder math questions you'll need to try it once, get stuck, stop and take a break. Then maybe when you try again later you'll have a new idea that may help!

For the problems in chapter 2, transformations are key to solving the problem.

For 2.12, note we are given $AB = CD$. This means it is possible to translate $\triangle PCD$ to a triangle $\triangle P'AB$ (so the base $CD$ is moved to $AB$). From here, see if you can use the triangle inequality to solve the problem.

As we see in 2.12, the key here is using the fact that $AB = CD$ as a hint for our transformation. To get started with 2.13, we know the opposite sides $AD = BC$, so try to do another translation to take advantage of this fact. For 2.14, sides $AB = AD$, so try a rotation here.

I'll let you think a little more about 2.15 and 2.16. If you're still stuck after you try a few more things, let us know and we can help more.