It's something to the second power. Let's test whether we have a change in concavity around x is equal to positive or negative 3. Remember, the numerator of our... Let me just rewrite our second derivative, just so you see it here. f prime prime of x. The numerator is this thing right here. It's 4x squared times 81 minus x to the fourth. The denominator was up here, x to the fourth plus 27 squared.