Why is a vertical tangent a place where it's hard to define our derivative? Well remember, our derivative, we're really trying to find our rate of change of y with respect to x. When you have a vertical tangent, you change your x a very small amount. You have an infinite change in y, either in the positive or the negative direction. So that's one situation where you have no derivative. And they tell us where we have a vertical tangent in here, where x is equal to three. So we have no, f is not differentiable at x equals three because of the vertical tangent.