Answer
$\color{blue}{9\ \text{cis}\ 180^\circ,\ 9\ \text{cis}\ \pi}$
Work Step by Step
$z=-9 = -9 +0i = x+iy \implies x=-9, y=0$
$\Huge\cdot$ modulus: $\quad r = \sqrt{x^2+y^2} = \sqrt{(-9)^2+0^2}= \sqrt{81} = 9$
$\Huge\cdot$ argument: $\quad \tan\theta=y/x=0/(-9)= 0;\ \cos\theta \lt 0 \implies \theta = 180^\circ \equiv \pi$ (smallest positive real angle $\theta$ from $+x$-axis to graph of $z$)
$\begin{array}{|c|c|c|} \hline
\text{Standard} & \text{Trigonometric} & \text{Trigonometric} \\
\text{Form} & \text{Form (deg)} & \text{Form (rad)} \\ \hline
-9 & 9\ \text{cis}\ 180^\circ & 9\ \text{cis}\ \pi \\ \hline
\end{array}$
