So now if we set u equal to seven x plus nine, do we have its derivative laying around? Well, sure, the seven is right over here. We know that du, if we want to write in differential form, du is equal to seven times dx. So du is equal to seven times dx. That part right over there is equal to du. And if we want to care about u, well, that's just going to be the seven x plus nine. That is our u.