Well, you've run into a weird part of Mathematics. The integral comes out to be . You might think that, since -2 is the cube root of -8 and then you square it that . But you'd be wrong. We must take that negative out and leave it on its own:

.

This is a similar rule to why you have to factor -1s out of the following: . This expression is not . This is

.

-Dan

In case it makes you feel better, playing around in the complex plane for a bit gives