Idon't understand pythagorean theorem. is there an easy way to figure out the answers

Ahaha, T.T Sadly there is no easy way to find out the answers, but I will explain it to you as simply as possible ^^~

A² + B² = C²

C² = The longest side or Hypotenuse
After that, just substitute the given values:

When you get the answer, it would have been squared  (n²) 'n' represents number you would have gotten~
 
So you just square root it, to find the original value~

Oh, and  if you are given another value instead of C² use this formulae~

B² = C² - A²
A² = C² - B

The pictures are from Google and my school website, I hope it helps and Good luck!

Ive seen this before! the answer is b