Converting this complex number to polar form?

Question

Given $ z = -1 - i$ ,I converted it to polar form, resulting r =$\sqrt 2$.

And $\theta = \tan^{-1} (\frac{-1}{-1}$) = 0.785 rads, which seems incorrect with the solutions of my instructor I don't understand why...

Answer 1

Hint:
In which quadrant of the plane does that point reside?

Answer 2

$-1-i$ is in the 3rd quarant, reasonable values for $\theta$ are between $\pi$ and $\frac32\pi$ (or $-\pi$ and $-\frac\pi2$ if you prefer values between $-\pi$ and $\pi$). $\tan^{-1}$ gives values between $-\frac\pi2$ and $\frac\pi2$, so you need to correct it by either adding or subtracting $\pi$.