Since the time series has trend and seasonal pattern, to forecast the quarterly sales of the textbook, the following regression equation was found: stack y t with overparenthesis on top equals 2120 space minus 620 space Q t r 1 space minus space 1320 space Q t r 2 space plus space 290 space Q t r 3 space plus space 24 space t , where Q t r 1 , Q t r 2and Q t r 3 are dummy variables corresponding to Quarters 1, 2 and 3, and t = time period. The sales in Quarter 4 of the next year are predicted to be