Here you are trying to find the hypotenuse. So you would take 5^2 (five squared) plus 12^2 (twelve squared) and add them using the equation a^2+b^2= c^2. Now you square 5 and 12 to get 25 and 144. You add them together and get 169. Take the square root of 169 and c. The square root on c will cancel out the squared (to the power of two) part leaving you with just c. And the square root of 169 is 13. So c (the missing length) is 13. C=13.