M7.

A plane flying at an altitude of 4.0 mi is at a direct distance D= (16.0+x^2) from an airport control tower, where x is the horizontal distance to the tower. If the plane's speed is 330 mi/h , how fast is D changing when x=6.4mi?

D is changing at a rate of ___ mi/h.

(Do not round until the final answer. Then round to the nearest ten as needed.)