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. And if this limit actually exists, we just call that value f prime of C. So that's just a review of differentiability. Now let's give ourselves a review of continuity. Con-ti-nuity.