Answer:
b = 21
Explanation:
By the Pythagorean Theorem, which states that if you have a right angle with legs of a and b, and a hypotenuse of c, then a^2+b^2=c^2. Use this on the right triangle in the diagram to get 841 = 400 + b^2, or 441 = b^2 and b = 21.
Note: I think that the 1st diagram was unnecessary. ;)