Answer
$r \cos (\theta) =4$
Work Step by Step
The conversion of polar co-ordinates $(r, \theta)$ to rectangular coordinates $(x,y)$ can be expressed as:
$x=r \ \cos(\theta)$, $y=r \ \sin(\theta) $
where, $r=\sqrt{x^2+y^2}$
We have: $x=4$
We substitute the conversion for $x$ above:
$ r \ \cos (\theta)=4$
