Answer
$${x^2} = - 12y$$
Work Step by Step
$$\eqalign{
& {\text{focus}}\left( {0, - 3} \right) \cr
& {\text{The focus is }}\left( {0,p} \right),{\text{ then the equation is of the form }} \cr
& {x^2} = 4py \cr
& {\text{with }}p = - 3 \cr
& {x^2} = 4\left( { - 3} \right)y \cr
& {x^2} = - 12y \cr} $$