A building has an entry the shape of a parabolic arch 74 ft high and 28 ft wide at the base, as shown below. A parabola opening down with vertex at the origin is graphed on the coordinate plane. The height of the parabola from top to bottom is seventy four feet and its width from left to right is twenty eight feet.

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