The directrix is a straight line that is found at the back of the opening of the parabola with a distance equal to its focal length. So, if the directrix is y = 3, that means that the focal length is 3 and the opening is downwards. The general equation for a parabola opening downwards is:
(x - h)² = -4a(y - k)
where a is the focal length and (h,k) is the vertex
We know a = 3. Now, since focus is at (-3,0) and directrix is at y=3, the middle of -3 and 3 is 0. So, the vertex is at the origin. Therefore, the equation would be:
x² = -12y
