its actually really easy, its called the pythagorean theorem, the formula is
[tex]{a}^{2} + {b}^{2} = {c }^{2} [/tex]
a and b are easy to find because the are the two sides that form the 90 degree angle, the one opposite to them is c. So let's do the one on the bottom left together, we have a and b, now we need to find c, plug the numbers into the formula...
[tex] {12}^{2} + {16}^{2} = {c}^{2} [/tex]
12 squared equals 144, 16 squared equals 256, 144 plus 256 equals 400. So at the moment, c squared still equals 400, but we just need c, not c squared. To get c by itself, we have to save root the 400, we end up with 20 as the end result, leaving 20 to be our final answer for c. Now sometimes you wont always have both sides of a or b, you'll have c and b, or c and a. In that case you would subtract, doing c squared minus a or b,( whichever one they give you)
