I am having trouble findig the roght excel formula to complete this problem. Can someone please help?

8. Suppose that you work for a U.S. senator who is contemplating writing a bill that would put a national sales tax in place. Because the tax would be levied on the sales revenue of retail stores, the senator has asked you to prepare a forecast of retail store sales for year 8 , based on data from year 1 through year 7 . The data are: (c)p8 [) a. Use the naive forceasting model presented in this chapter to prepare a forecast of retail store sales for each year from 2 through 8 . b. Prepare a time-series graph of the actual and foreeast values of retail store sales for the entire period. (You will not have a foreenst for year 1 or an actual value for year 8 .) c. Calculate the MAPE for your forecast series using the values for year 2 through year 7