A building has an entry the shape of a parabolic arch 96 ft high and 18 ft wide at the base, as shown below. A parabola opening down with a vertex at the origin is graphed on the coordinate plane. The height of the parabola from top to bottom is ninety-six feet and its width from left to right is eighteen feet.

Find an equation for the parabola if the vertex is put at the origin of the coordinate system.