The pilot of a 90,000-lb airplane which is originally flying horizontally at a speed of 400 mi/hr

Question:

The pilot of a 90,000-lb airplane which is originally flying horizontally at a speed of 400 mi/hr cuts off all engine power and enters a 5° glide path as shown. After 120 seconds the airspeed is 360 mi/hr. Calculate the time-average drag force D (air resistance to motion along the flight path).