Well, to figure that out, you have to say, well, what is f prime of x? F prime of x is just going to be equal to two. And then test the equality. Is two, is f prime of x equal to f of x, which is two x, minus x, minus x. And so let's see, we are going to get two is equal to x. So you might be tempted to say, oh, hey, I just solved for x or something like that. But this would tell you that this is not a solution because this needs to be true for any x that is in the domain of this function.