Can you help me understand this Excel question?

Multiple Regression

For this assignment, you are required to complete Problem 64 (p. 593) in Chapter 12 of your textbook. Use the appropriate Excel file templates from www.cengagebrain.com. Once complete, post your Excel Document in Waypoint. Show all work.

23-64) Let Yt be the sales during month t (in thousands of dollars) for a photography studio, and let Pt be the price charged for portraits during month t. The data are in the file Week 4 Assignment Chapter 12 Problem 64. Use regression to fit the following model to these data:
Yt = a + b1Yt?1 + b2Pt + et
This equation indicates that last month’s sales and the current month’s price are explanatory variables. The last term, et, is an error term.

1. If the price of a portrait during month 21 is $10, what would you predict for sales in month 21?
2. Does there appear to be a problem with autocorrelation of the residual? Explain your answer.