3. Find an equation of a parabola with a vertex at the origin and directrix y = –4

We have the equation that

y =  [1 / (4p) ] x^2      where p  = the distance from the vertex to the origin = the distance from the vertex to the directrix.....so   p  = 4  and we have that

y = [1 /(4 *4) ]  x^2

y = ( 1/16)x^2