What formula do you use to prove that a triangle is a right triangle?

If you have the length of each side, apply the pythagorean theorem to the triangle. If you get a true statement, then you can be sure that you do indeed have a right triangle. If you get a false statement then you can be sure that your triangle is not a right triangle. 
Pythagorean therem is : A squared + B squared = C squared

The answer is, the streetlight is 18 inches tall.