Hi and welcome to the KH forum!
It sounds like the beginning of the armhole and the sleeve cap shaping. After the cast off of 3sts at the beg of the next 2 rows, call the next row, row 1. You would decrease at each end of rows 1,2,3,5 and 9. That'll take you from 49sts to 39sts as the patterns says. Good luck with finishing up the sweater.