Geometric mean between 9 and 36

I've actually just learned this. We would set up the proportion as X/9=36/X so after we cross multiply we get x^2=324. Then we find the square root of both sides to simplify. And using my calculator the square root of x^2 is just x. And the square root of 324 is 18. So the final answer is x=18 or the geometric mean is 18.