Question 449573
As you know the directrix of the parabola {{{x^2=4cy}}} is y=-c, since y=3, we get

3=-c or c=-3, and the equation of parabola will be: {{{x^2=-12y}}}.

{{{graph(300, 300, -5, 5, -5, 5, (-1/12)*x^2)}}}.