geometry radicals

I have been very confused on radicals for the past week and I am on a bit of a time crunch to finish my second semester in geometry before the new school year starts. Can someone please explain how radicals work step by step and how I can find the simplest radical form? I will put two questions that I am stumped on below:

1. In a right triangle, the short leg = 6 and the hypotenuse = 8. Find the length of the longest leg (x).
2. Write √72 in simplest radical form.

I know the answers because I was given them after submitting the quiz, but I still don't know what steps I should take to get the answers.