Pythagorean Theorem with a Square Root

I know this is really basic, but I keep getting hung up on the square root in this problem:

I have: 9^2 + b^2 = (9√2)^2

How is the RHS simplified? If you subtract 9^2 first (assuming that would be the best way to approach this), and then take the square root of both sides, I'm not sure what the process is to simplify the radical that is already in place....