CHER PRACTICE ANOTHER Watching a Rocket Launch At a distance of 3,000 ft from the launch site, a spectator is observing a rocket being launched. Suppose the rocket lifts off vertically and reaches an altitude of x ft (see the accompanying figure). Rocket Spectator Launching pad 3000 ft (a) Find an expression giving the distance between the spectator and the rocket. D = (b) What is the distance (in feet) between the spectator and the rocket when the rocket reaches an altitude of 21,000 ft? (Round your answer to the nearest foot.) ft