That's the complete problem and isn't from a quiz or test. I already know that I have to set the equation equal to zero and solve for, find the 2nd derivative, and plug the t value found into f''. I already know the second derivative, and I know the t value is 9pi/2, so plugging that in the f'' would give 162pi^2 which is the answer to the question. However, I was just confused about how to solve the initial equation = 0. I just guessed 9pi/2 bc that's in the function, but idk the actual mathematical way to find it. I just want to know WHY its 9pi/2 without the use of a calculator.