Hey guys, I'm having trouble understanding an aspect of u-substitution. In this problem:

Integrate:

u =

So that takes care of the in the first equation, and the rest of the integration is easy.

But I don't understand why the same principle can't be applied here:

Integrate:

u = x-3

du = x * dx

Why can't you cancel out x * dx with du here?

Thanks in advance for any help.