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. Your second derivative is 0 when x is equal to 0, but it is not an inflection point, because notice the concavity did not change around 0. Our second derivative is positive as we approach 0 from the left, and it's positive as we approach 0 from the right. In general, at 0, as we're near 0 from either direction, we're going to be concave upwards.