So what is the Pythagorean Theorem:
⇒ it states the sum of both leg's lengths squared is equal to
⇒ hypotenuse's length squared
(look at the image attached for a better explanation)
To find out
, let's look to the Pythagorean equation to see what we are missing:

- a = 10 ⇒
= 100

So b^2 = 225 ⇒ Choice B
Hope that helps!