Divide the root number (15) by 2, then subtract 1/2 from the result. Square the result and that number is x. 15 divided by 2 = 7.5, Subtract .5 and get 7, square 7 to get 49. 49 is x, works every time with this equation no matter what the root is. x + 15 = 64. sqrt of 64 = 8, sqrt of 49 = 7, 8 + 7 = 15.