An air traffic controller spots two airplanes at the same altitude converging to point A as they fly at right angles to each other. One airplane is 180 miles from point A and has a speed of 720 miles per hour. The other is 270 miles from point A and has a speed of 1080 miles per hour.

At what rate is the distance between the two planes changing? Think carefully about whether this rate should be positive or negative.
The distance is changing at what miles per hour.