Use the standard form for a parabola:

where are the x,y coordinates of the vertex of the parabola, and p is the distance from the vertex to the focus of the parabola. The best place to put the wtaer is at the focus. Given the information that you have, you know that the parabola passes through the coordinates (-3,1), (0,0), and (3,1), so you should be able to determine the value for p.