Answer
$$
r =-\frac{3}{2+ \cos \theta}.
$$
Work Step by Step
The polar equation of the conic section of eccentricity $e \gt0$, with focus at the origin and directrix $x = d$ is
$$
r=\frac{e d}{1+e \cos \theta}.
$$
Now, since $e=1/2$ and $x=-3$, then the polar equation is
$$
r=\frac{-3/2}{1+(1/2) \cos \theta}=-\frac{3}{2+ \cos \theta}.
$$