You can use the Pythagorean theorem because this is a right triangle:
a and b are the legs and c is the hypotenuse. The hypotenuse is always the longest side in a right triangle (since it is opposite the biggest angle). The two legs squared have to add up to the hypotenuse squared.
Fill in values:
Simplify exponents:
Isolate the variable. Subtract 225 from both sides: