A right triangle's formula, according to Pythagorean's Theorem, is a² + b² = c². a and b are the two legs, and c is the hypotenuse. To solve, we must substitute the variables with the given measurements.
9² + 12² = 15²
Let's now solve:
81 + 144 = 225
225 = 225
The formula works out in the end, so it is a right triangle!
