How would you integrate something like
sqrt(9-y)*y dy from 0 to 9
The problem is that "9- y" inside the square root, right? So try u= 9- y. Then du= -dy. and, of course, y= 9- u. When y= 0, u= 9 and when y= 9, u= 0.
So the and the integral will be from 9 to 0. Swapping the limits of integration gets rid of that negative.