The hypotenuse is 30, opposite is 24, and adjacent is 18. How do I get the degree of 18?

"A 40 foot ladder is used to reach the top of a 24-foot wall. If the ladder extends 10 feet past the top of the wall, find the measure of the angle that the ladder forms with the horizontal ground. (Answer to nearest whole degree.)" So my answer is square root of 324. It is equal to 18. I'm confused as to how do I get the answer nearest whole degree? Is 18 the final answer?