At the skateboard park, the hot new attraction is the U-Dip, a cement structure embedded into the ground. The cross-sectional view of the U-Dip is a parabola that dips 15 feet below the ground. The width at ground level, its widest part is 40 feet across. Sketch the cross-sectional view of the U-Dip, and find an equation of the parabola that models it.