Answer
$r=\dfrac{5}{1-\sin \theta}$
Work Step by Step
From the given statement , we have: Directrix , $a=5$ and eccentricity ; $e=1$
The equation must be of the form as follows: $r=\dfrac{ae}{1-e \sin \theta}$
Then, we have $r=\dfrac{(1)(5)}{1-(1) \sin \theta}$
Therefore, $r=\dfrac{5}{1-\sin \theta}$
