Answer
The polar equation of $x=7$ that expresses $r$ in terms of $\theta $ is $r=\frac{7}{\cos \theta }$.
Work Step by Step
The relation between polar coordinates and rectangular coordinate $x$ can be expressed as below:
$x=r\cos \theta $ (2)
Substitute the value of $x\ $ from equation (2) into equation (1) to get,
$\begin{align}
& x=7 \\
& r\cos \theta =7 \\
& r=\frac{7}{\cos \theta }
\end{align}$
