Answer:
y^2 = -12x
Step-by-step explanation:
For a parabola whose vertex is at (0,0), its standard equation is

where the directrix is given by x = a.
Now in our case, the vertex of our parabola is at (0,0) and the directrix is x = 3. This tells us that a = 3 and so the above equation gives


which is our answer!