Aplane at an altitude of 2 feet starts to descend for landing after flying for six hours. the entire flight time was 6 hours 40 minutes. determine the average rate of descent of the plane

Check the picture below.

so, let's see, the plane departs from (0, 0), the origin, goes up to 20000 feet, sticks around flying for 6 hours, 6hrs = 3600 minutes, (6*60).

so the plane's descent starts off at the point (3600, 20000) down to (3640, 0) as you see in the picture, keeping in mind that average rate of change = slope.

the slope is negative, since it's a descent, the plane is coming down, not going up over the minutes axis.