And so u-substitution is all about, well, do we see a pattern like that inside the integral? Do we see a potential inside function, a g of x, where I see its derivative being multiplied? Well, we see that over here. If I look at x squared plus x, if I make that the u, what's the derivative of that? Well, the derivative of x squared plus x is two x plus one. So we should make that substitution. If we say u is equal to x squared plus x, then we could say du dx, the derivative of u with respect to x, is equal to two x plus one.