Answer
$$z=\sqrt{2} \left ( \cos \frac{5\pi }{4} +i \sin \frac{5\pi }{4} \right ).$$
Work Step by Step
We plot the complex number $z=-1-i$ the same way we plot $(-1,-1)$ in the rectangular coordinate system.
Please note that the complex number $z=a+bi$ is written in polar form as$$z=r(\cos \theta +i \sin \theta ),$$where $a=r\cos \theta$, $b=r\sin \theta$, $r=\sqrt{a^2+b^2}$, and $\tan \theta = \frac{b}{a}$, $0 \le \theta \lt 2\pi $.
So, for the complex number $z=-1-i$, we have$$r=\sqrt{(-1)^2+(-1)^2}=\sqrt{2}, \\ \tan \theta =\frac{-1}{-1}=1.$$Since $\cos \theta =\frac{a}{r} = \frac{-1}{\sqrt{2}} \lt 0$, the argument, $\theta$, must lie in quadrant $III$. So$$\theta = \frac{\pi }{4} +\pi = \frac{5\pi }{4}.$$Thus, the complex number has the polar form$$z=\sqrt{2} \left ( \cos \frac{5\pi }{4} +i \sin \frac{5\pi }{4} \right ).$$
