A small aircraft starts its descent from an altitude of h = 4/5 miles, 4 miles west of the runway (see figure).(a) Find the cubic function f(x) = ax³ + bx² + cx + d on the interval [-4, 0] that describes a smooth glide path for thelanding.(b) The function in part (a) models the glide path of the plane. When would the plane be descending the greatest rate?