The answer is square root of 175. To get the answer first you have to first have the equation a^2+b^2=c^2. So your new equation would be a^2+15^2=20^2. Since 20^2 equals to 400 and 15^2 equals to 225. All you have to do is 400-225 which is 175. Since there is no two same numbers that can multiply to get 175 you leave it as square root of 175.
