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You're going to work one shoulder and leave the rest of the sts for the center and the other shoulder for later.
K12, turn as you would if you were at the end of a row. Place the sts remaining on the left needle on a holder or spare needle.
Purl the 12sts that you just knit.
Knit across 9sts to last 3 sts, k2tog, k1
Turn to the purl side, and purl 11 sts to the end of the row.
Directions should follow for shaping the shoulder on these 11sts.