Geometry: Common Core (15th Edition)

The Pythagorean Theorem states that for any right triangle, $a^{2} + b^{2} =c^{2},$ where c is the length of the hypotenuse and a and b are the length of the other two sides. Plugging in the dimensions of the given triangle into the Pythagorean Theorem, we obtain: $$x^2 + 9^2 = 15^2 \\ x^2 +81 =225 \\ x^2=144 \\x=12$$