Pythagorean theorem: a^2 + b^2 = c^2
C represents the hypotenuse (the longest side) the one we are figuring out
Use Pythagorean theorem:
9^2 + 12^2 = c^2
81 + 144 = c^2
225 = c^2
(Square root both sides to get c by itself, by getting rid of the ^2 (the squared symbol))
The square root of 225 is 15, so,
15=c