You should really view this as the product of three things. Oftentimes, people just view the dx as somehow part of the integral operator, but you could rearrange it. This would actually be legitimate. You could say the integral of the square root of x squared plus x times two x plus one dx. And if you wanted to be really clear, you could even put all of those things in parentheses or something like that. And so here, this is our u, and this right over here is our du. And so we could rewrite this as being equal to the integral of the square root of u, because x squared plus x is u, times du, which is much easier to evaluate.