You give me an x, you give me an arbitrary x where the derivative is defined. I'm going to plug it into this. Whatever this ends up being, it might be some nice clean algebraic expression. And then I'm going to give you a number. So for example, if you wanted to find, you could calculate this somehow. Or you could even leave it in this form. And then if you wanted f prime of a, you would just substitute a into your function definition.