And since it's concave downwards, that implies that our critical point that sits where, during where the interval is concave downward, that critical point is a local maximum, is a maximum. So this is the x value at which our function attains our maximum. Now, what is that maximum value? Well, we could type that back in into our original expression for volume to figure what that is. So let's figure out what the volume, when we get to 3.92, is equal to. What is our maximum volume? So get the calculator back out.