Use the Pythagorean theorem. a^2 + b^2 = c^2 So you already know one of the legs (we'll call it 'a') is 21. And the hypotenuse (this is 'c') is 53. Substitute these values into the theorem. 21^2 + b^2 = 53^2 441 + b^2 = 2,809 b^2 = 2,368 b = the square root of 2,368. This simplifies to 8 * the square root of 37. This is approximately 48.66.