Verified Answer :

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.