But it is often useful to kind of pretend that it is a fraction. And you could kind of view this if you wanted to just get a du, if you just wanted to get a differential form over here. How much does u change for a given change in x? You could multiply both sides times dx. So both sides times dx. And so if we were to pretend that they were fractions, and it will give you the correct differential form, you're going to be left with du is equal to, du is equal to three x squared plus two x dx. Now why is this over here?