Answer
The required equation in the polar form is $r=-7\,\csc \,\theta $.
Work Step by Step
We know that $x=r\,\cos \,\theta \ \text{ and }\ y=r\,\sin \,\theta $.
Put $x=r\,\cos \,\theta \ \text{ and }\ y=r\,\sin \,\theta $ in the provided equation $y=-7.$
Therefore,
$\begin{align}
& y=-7 \\
& r\,\sin \,\theta =-7 \\
& r=-7\times \frac{1}{\sin \,\theta }
\end{align}$
That is, $r=-7\,\csc \,\theta $.
