Why do the plates on Earth’s surface move?

Tectonic plates, although rigid, also move. And they move in rebound, because of the tremendous heat that comes out of the interior of our planet, which burns at 6.7500º C, almost like the sun. This fiery heat moves the mantle and consequently expands into the lithosphere and Earth's crust.

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