The form of the equation of a parabola that opens up or down that I like to use is this:
In this form, the vertex, is (h,k), and p is the distance from the vertex to the focus and from the vertex to the directrix. With that definition for p, 4p is the length of the latus rectum.
In your problem the vertex and the length of the latus rectum are given, so you just need to plug in the values -- remembering that the coefficient 1/4p is negative because the parabola opens downward.
