Find the polar represenatation z= -1 + i.

Solution: |z| = $\displaystyle \sqrt{2}$ and $\displaystyle \theta = \frac{3\pi}{4}$. Thus,

-1 + i = $\displaystyle \sqrt{2}$[ $\displaystyle cos\frac{3\pi}{4} + i sin\frac{3\pi}{4}$]

It's been awhile since I have taken calculus, let alone any geometry. Could someone please explain to me how to find the $\displaystyle \theta$ in this answer?

A walkthrough of another problem could help me:

Find the polar representation for: $\displaystyle (2-i)^2$

Thanks for any help!