What happens when x is slightly below 0? Not all of them necessarily, but if x is like 0.1, what is the second derivative going to be doing? If x is minus 0.1, this term right here is going to be positive, and then this is going to be 81 minus 0.1 to the 4th. That's going to be a very small number. It's going to be some positive number times 81 minus a small number, so it's going to be a positive number. When x is less than 0, or just slightly less than 0, our second derivative is positive. What happens when x is slightly larger?