It is the longest side, directly across from the right angle. You can
calculate it knowing a side and an angle (which gives you both non-right
angles) or more typically by using the Pythagorean Theorem, which gives
the formula.
Since C is 61 degrees, A is 90 degrees because of the right angle, and B would be 29 degrees because you add 61 and 90 together and subtract that amount from 180.
Wait I'm sorry if this is what you aren't looking for-
