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.
