Answer
$\dfrac{3}{1- \cos \theta}$
Work Step by Step
The polar equation of a conic with eccentricity $e$ and directrix $x=d$ can be written as:
$r=\dfrac{ed}{1-e \cos \theta}$ ....(1)
Given: Directrix , $x=-3$
The conic will be a parabola when $e = 1$
Thus, the equation (1) can be written as:
$r=\dfrac{ed}{1-e \cos \theta}=\dfrac{(1)(3)}{1-(1)\cos \theta}=\dfrac{3}{1- \cos \theta}$
