the answer would be the third option, "no, ariel should have added 9^2 and 12^2 and compared that to 15^2."
this is found through the pythagorean theorem, which is a^2 + b^2 = c^2.
this is the correct way of solving the problem:
9^2 + 12^2 = 15^2
81 + 144 = 225
225 = 225