So once again, pause the video, see if you can figure it out. All right, now let's inspect Katie's work. So she's taking the derivative of this. And let's see, over here it looks like she's taking the derivative of the entire expression with respect to the inner expression. And that is close to applying the chain rule properly, but it's not applying the chain rule properly. So her work is not correct, and her mistake is she's not correctly applying the chain rule. Just as a review, the chain rule says, look, if we're trying to take the derivative with respect to x of f of g of x, f of g of x, f of g of x, that this is going to be equal to, this is going to be equal to the derivative of the whole thing with respect to g of x.