Answer
(a) See the graph.
(b)
$r=20$
$θ=\pi~~$
(c)
$z=20(cos~\pi+i~sin~\pi)$
Work Step by Step
(b)
$r=|z|=\sqrt {a^2+b^2}=\sqrt {(-20)^2+0^2}=\sqrt {400}=20$
$tan~θ=\frac{b}{a}=\frac{0}{-20}=0$
The point lies in the negative real axis. So:
$θ=\pi~~$
(c)
$z=r(cos~θ+i~sin~θ)$
$z=20(cos~\pi+i~sin~\pi)$
