Explanation:
the standard form is (x - h)² = 4p(y - k), where the focus is (h, k + p) and the directrix is y = k - p.
the vertex is exactly in the middle between focus and directrix, and is therefore (h, k) = (0, 0).
therefore, p = 9.
(x - 0)² = 4×9×(y - 0)
x² = 36y
y = 1/36 x²