So let's figure out what, at either of these critical points. And it'll all fit together. Remember, if it's concave upwards, then we're kind of in a U shape. If it's concave downwards, then we're in a kind of upside down U shape. So f prime prime, our second derivative at x is equal to 0 is equal to what? It's equal to 36 zero squared minus 24 times 0. So that's just 0.