So this is true if and only if F prime prime of x goes from, goes from positive, positive to negative, negative, or vice versa, or vice versa. So where do we see F prime prime of x going from positive to negative? Well that's going to be true, that's going to be true if and only if F prime of x goes from being increasing to decreasing, or vice versa, F prime goes from increasing, increasing to decreasing, decreasing, or vice versa. I'm using a lot of vice versa here. So now let's, and I wanted to think of it in terms of F prime because we have the graph of F prime. So F prime goes from increasing to decreasing, or vice versa, or we could go from decreasing to increasing. Well let's think about it.