Answer
$x^2=12y$
Work Step by Step
We are told that the directrix is below the given vertex; thus, $ p$ is the positive of the distance between the two points.
So, $p=3$
Since the vertex and focus have the same y-coordinate, the parabola must have a horizontal axis.
Now, we will write the equation for a parabola that has a vertical axis using the vertex and $p$.
$x^2=4py \implies x^2=(4)(3) \times y $
$\implies x^2=12y$
