Find the equation of a parabola with vertex (3, 5) and directrix y = 7

Keeping in mind that, the vertex is "p" distance from either the focus or directrix, in this case the directrix is above the vertex, so that means the parabola is a vertical parabola and also that it opens downwards, like the one in the picture below.

since the directrix is above the vertex, the focus point  is on the opposite direction "p" units from the vertex, as you see in the picture, at (3,3).

now, the parabola is opening downwards, therefore "p" is negative.