A plane flying at an altitude of 3.0 mi is at a direct distance D = 19.0 + x" from an airport control tower, where x is the horizontal distance to the tower. If the plane's speed is 340 mi/h, how fast is D changing when x = 7.4 mi? . . . . . D is changing at a rate of mi/h. (Do not round until the final answer. Then round to the nearest ten as needed.)