I read it as cast on 17 stitches to the left needle. Then purl 3tog and purl 15 stitches (the ones you cast on).
You'll end up with 16 stitches on the right needle and should have 25 on the left.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry