SOLUTION
The equation of the parabola is defined using
[tex]y=\frac{1}{4(f-k)}(x-h)^2+k[/tex]Since the vertex is at the origin the equation becomes
[tex]\begin{gathered} y=\frac{1}{4(f-0)}(x-0)^2+0 \\ y=\frac{1}{4f}x^2 \end{gathered}[/tex]Since the directrix is y=5
The distance from the focus to the vertex is equal to the distance from the vertex to the directrix
It follows
[tex]\begin{gathered} f-k=k-5 \\ f=-5 \end{gathered}[/tex]Thus the equation becomes
[tex]\begin{gathered} y=\frac{1}{4\times-5}x^2 \\ y=-\frac{x^2}{20} \end{gathered}[/tex]Therefore the equation of the required parabola is
[tex]y=-\frac{x^2}{20}[/tex]