So how is the second derivative going to help us think whether that was an overestimate or an underestimate? Well, the second derivative can help us know about concavity. It'll let us know, well, is our slope increasing over this interval, or is our slope decreasing? And then we can use that to estimate whether we overestimated or not. So first, let's just find the second derivative. So we have the first derivative written up here. Let me just rewrite it, and I'll distribute the negative 1 4th, because it'll be a little bit more straightforward then.