You have a right triangle. The slide is 30 feet long, so that's the hypotenuse. The base is 18 ft. You can use the Pythagorean theorem to find the height. If you call the height x, then solve x^2+18^2=30^2. Since you're finding a distance, when you take the square root of 30^2 - 18^2, you won't have to take the negative root.