Knowing the directrix and focus and using the definition of parabola to derive the equation will give you, if you so work it, an equation in that form. The vertex is (h,k); and the focus point and the directrix line are units from the vertex. As the form goes, the equation is for a parabola with vertical symmetry axis. If p is positive, the focus is above the vertex; and if p is negative, focus is below the vertex.