Answer
$r=\frac{3}{\sin \theta }$.
Work Step by Step
Rectangular equation is $y=3$ …… (I)
The relation between polar coordinates and rectangular coordinate $y$ can be expressed as below:
$y=r\sin \theta $ …… (II)
Substitute values of $x\ $ from (II) in (I) to get
$\begin{align}
& y=3 \\
& r\sin \theta =3 \\
& r=\frac{3}{\sin \theta }
\end{align}$
Hence the obtained polar expression is $r=\frac{3}{\sin \theta }$.
