Calculate $4-$quarter moving average values for this time series $($column E$).$

Calculate centered moving average values and seasonal values $($column $F$ and $G).$

Calculate seasonal indexes $($column J$)$ for the four quarters.

Copy seasonal indexes $($column $J)$ to column $O,$ and calculate de$-$seasonalized number sold $($column P$).$

Construct a time series plot and develop linear trend equation using the de$-$seasonalized data $($without seasonal components$).$

Calculate trend forecast in year $8$ based on the linear trend equation.

Copy seasonal index and compute quarterly forecast values $($re$-$seasonalize$).$