This is Pythagorean Theorem. AB is the "a" side, BC is the "b" side, and the x (or AC) is the hypotenuse. In Pythagorean Theorem, a^2 + b^2 = c^2 is used to find the hypotenuse, or in this case, segment AC. We find this by doing 8^2 + 15^2 which gives us 289. We then take the square of that which is 17 to find the hypotenuse, or the length of line segment AC
