it's asking you to find the derivative, which is -0.06x+1.8, and then, when it equals zero, it has reached the top. that is, -0.06x+1.8=0, or x= 30. plug that into the original formula, and you get -0.03(30)²+1.8(30)-24, or 3. the answer is 3 (inches, presumably)
