Earth’s mantle is
a. a layer of molten metal.
b. a layer of hot rock.
c. a dense ball of solid metal.
d. a layer of rock that forms earth’s outer skin.

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