So we could make the substitution. We could say that u is equal to the natural log of x. And the reason why I picked natural log of x is because I see something, I see its exact derivative here, something close to its derivative. In this case, it's its exact derivative. And so then I could say du, dx, du, dx is equal to one over x, which means that du is equal to one over x, dx. And so here you have it. This right over here is du, and then this right over here is our u.