So, in your typical substitution problem, you're looking to substitute u for the composite function (the function within a function). In this case, that'd be x-1, and eventually, that'll end up being u7 when you make the substitution.
dx wouldn't be an issue here, either, because if u = x-1, then du is just dx.
But you still have the other piece floating around. x2+2. Normally, when you do a u-substitution, the other piece ends up being the derivative of u, so it disappears. But that didn't happen here, because du was just dx, not anything with an x2 in it. What to do, what to do. :)
Well, if u = x-1, then what's x in terms of u? Can you substitute that for x, simplify, and end up with an integral you can handle?
Does that help?