If we are finding the long angled side (the hypotenuse), we must keep in mind the Pythagorean theorem. This states that in order to find the answer, we must follow: A squared + B squared = C squared. A and B are known as the legs and we use these for A and B. After squaring both 15 and 8 we get 225 + 64= 289. Now that we have our answer of 289, we can square this to find our hypotenuse which is 17. To find the square root, you multiply different numbers by themselves until you reach your answer. 17 x 17 = 289 so the hypotenuse is 17 centimeters long. I hope this helps! :)