Answer
$$r = \frac{9}{1 + 3\space cos(\theta)}$$
Work Step by Step
1. Determine the equation that we are going to use:
Since the directrix is defined by $x = 3$, we are going to use $cos(\theta)$ in the formula.
Since $3$ is positive, the equation will have "$+cos(\theta)$":
$$r = \frac{ed}{1 + ecos(\theta)}$$
2. Substitute the given values for $d$ (directrix) and e (eccentricity):
$$r = \frac{(3)(3)}{1 + (3)cos(\theta)} = \frac{9}{1 + 3\space cos(\theta)}$$
