So if you wanted to go from here to there, you could kind of pretend that you're multiplying both sides by dx. But these are equivalent statements, and we want to get it in differential form in order to do proper u-substitution. The reason why this is useful, and I'll just rewrite it up here so that it becomes pretty obvious, our original integral we can rewrite as 4x to the third dx over x to the fourth plus 7. Then it's pretty clear what's du and what's u. u, which we set to be equal to x to the fourth plus 7, and then du is equal to this. It's equal to 4x to the third dx. We saw it right over here. So we can rewrite this integral, and I'll try to stay consistent with the colors, as the indefinite integral.