You're going to like this part. Since
we know immediately that:
In other words, if ex is its own mother, then it's its own child, too.
Now, we can find lots of other integrals using substitution. For instance,
(You know, u=3x, du=3dx, etc. Usual drill.)
More generally, it's worth remembering that:
We could also use substitution or eyeball to see that