Oh no, not another one of Q's easy but slightly tricky quiz questions. Today it's regarding finding the shortest distance between a point and a line. This explains the theory behind it, and how to calculate it.
But if you are only interested in the distance between P and P3 itself, and you don't care what P is, this math is moderately wasteful, as you have to first find P, then calculate the distance between P and P3. Surely there must be a way to calculate the distance without ever calculating P. And in fact, there is.
Find it, without cheating (looking up the answer online of through friends. It's actually pretty easy and simple. I just thought it was kind of neat.