The next line after 1048 = 1657 + (20)d should be 20d = 1048 - 1657.

There is also a remark about the number of years. If T1 means the number of beekeepers in 1986, then we count year 1986 as number 1. Then year 2007 would be number 2007 - 1986 + 1 = 22. Why +1? E.g., year 1987 would be number 2 and 1987 - 1986 = 1. Therefore, n = 22 and you need to divide the difference in the number of beekeepers by n - 1, i.e., 21.