Quick imaginary number question

[Stojakapimp]

I'm trying to do my engineering homework but i'm a little confused on a small detail. In my equation, I have (iw)^2. Now does this become -w or +w? It seems to me that it would be -w, but I think my professor has written it down as +w before. Anyone know for sure?

 

[Anubis]

(iw)^2=i^2w^2=-w