Answer:
4 area codes per year.
Explanation:
There were 171 area cods in the United states in year 1996.
In 2007 there were 215 area codes. We have to find the rate of change from 1996 to 2007.
Since rate of change is defined by R =

=

=

= 4 area codes per year
Therefore, rate of change is 4 area codes per year.