The question says working capital will need to be increased annually , but in the solution it’s decreasing. (9600 is year 0, 2112 in year 1, 722 in year 2 and 1316 in year 3).
It is not decreasing. The 2,112 in year 1 is the extra working capital needed (and brings the total up to 9,600 + 2,112). The same applies in the later years.