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.