We have critical points at x = 0 and .We can't use the second derivative test at x = 0 because f'(0) doesn't even exist. The first derivative test it is. Here's the numberline, and the values we need to plug into f':
These next ones we'll do with a calculator. If you want to do them by scratch we recommend using x = ⅛ instead of x = 0.25, since ⅛ has a nice cube root. Remember, all we care about is whether we find positive or negative numbers.