Well, it's going to be change in y over change in x. The change in y is f of x minus f of C. That's our change in y right over here. And this is all a review. This is just one definition of the derivative, or one way to think about the derivative. So it's going to be f of x minus f of C, that's our change in y, over our change in x, which is x minus C. It is x minus C. So if this limit exists, then we're able to find the slope of the tangent line at this point. And we call that slope of the tangent line, we call that the derivative at x equals C. We say that this is going to be equal to f prime of C. All of this is review. So if we're saying, one way to think about it, if we're saying that the function f is differentiable at x equals C, we're really just saying that this limit right over here actually exists.