Analytic Geometry(Continued)

First of all, in math, we did mental math. Unfortunately, today was not my best day, and one of the reasons it wasn't so good is because I don't really understand what we're learning right now. We're learning about ways to find the distance between points. We continued the notes from yesterday, and now I understand how to do the notes from yesterday, but I don't really understand what we did today. I did ask somebody else to show me, and it made a bit more sense. However, I still don't fully understand it.