You drive 4 miles from your house to your friend's house. your house is at an elevation of 622 feet. your friend's house is at an elevation of 486 feet. what is the mean change in elevation per mile?

Just subtract 622 feet from 486 feet to get the change in elevation.  The result will be negative because your friend's house is at a lower elevation than yours.

Now find the mean change in elevation per mile by dividing (486 ft - 622 ft) by 4 miles:
                      -136 ft
                     = -34 ft/mile
                       4 mi