If you read the problem attentively, you will see these words "the number x of years after 1996"  there.


These words mean that you should write 0 for 1996 and 10 for 2006.


After that, you can continue your solution in the formal way.


                   +-----------+
                   |   B U T   |
                   +-----------+


The problem can be easily solved mentally without using complicated interpolation-extrapolation formula.


From the problem, the time interval is 10 years; the difference in percentage is 22.2-12.2 = 10 percents total.


It tells us that the average decreasing rate is 1 percent per year.


So we write  y = 22.2 - x  for percents, where  x  represents years after 1996 to 2006.