<===== Vertex form of a parabolic equation ------- Substituting (0, - 20) for (x, y) and (- 3, 16) for (h, k) - 20 = 9a + 16 - 20 - 16 = 9a - 36 = 9a <===== Vertex form of a parabolic equation ------- Substituting 0 for y, - 4 for a, and (- 3, 16) for (h, k) (x + 5)(x + 1) = 0 x = - 5 OR x = - 1