When x is right above 0, what happens? Let's say x was 0.01, or 0.1, positive 0.1. It's going to be the same thing, because in both cases we're squaring and we're taking the 4th, so you're kind of losing your sign information. If x is 0.1, this thing is going to be a small positive number. You're going to be subtracting a very small positive number from 81, but 81 minus a small number is still going to be positive. You're going to have a positive times a positive, so your second derivative is still going to be greater than 0. Something interesting here.