We are given that the focus (0, –3) lies on the y-axis, therefore y-axis is the axis of parabola. Also the directrix is y = 3 i.e. y = a and focus (0, –3) i.e. (0, –a). The equation of parabola is of the form x2 = –4ay.
The required equation of parabola is x2 = – 4 × 3y ⇒ x2 = –12y.