This is the Pythagorean theorem, that equation is a^2 + b^2 = c^2, but if you dont have b you can manipulate the equation, that equation is c^2-a^2=b^2. So you take 15^2 - 12^2 and that equals 81, then you take the square root of 81, which equals 9, so your answer would be 9 feet. Hope that helped :)
