Let me draw a line just so we're doing something different. So let's take, let us take the limit as x approaches C of f of x, of f of x minus f of C. Of f of x minus f of C. Well can we rewrite this? Well we could rewrite this as the limit as x approaches C. And we can essentially take this expression and multiply and divide it by x minus C. So let's multiply it times x minus C. x minus C and divide it by x minus C. So we have f of x minus f of C. All of that over x minus C. So all I did is I multiplied and I divided by x minus C. Well what's this limit going to be equal to? This is going to be equal to, it's going to be the limit, and I'm just applying the property of limit, property, I'm applying a property of limits here. So the limit of the product is equal to the same thing as the product of the limits. So it's the limit as x approaches C of x minus C times the limit, let me write it this way, times the limit as x approaches C of f of x minus f of C. All of that over x minus C. Now what is this thing right over here? Well if we assume that f is differentiable at C, and we're going to do that, actually I should have started off there.