Of f of x. And the other way to denote the derivative of f of x is to just say that this is the same thing. This is equal to a times, this thing right over here is the exact same thing as f prime of x. Now, this might all look like really fancy notation, but I think if I gave you an example, it might make some sense. So what about if I were to ask you the derivative with respect to x of 2 times x to the fifth power? Well, this property that I just articulated says, well, this is going to be the same thing as 2 times the derivative. This is going to be the same thing as 2 times the derivative of x to the fifth.