Next row (RS): k2, sl1, k1, psso, K to last 4 sts, K2tog, k2. 40sts.

Was your decrease row. Now you are supposed to decrease one stitch at each end in exactly the same way you did on that decrease row on the 6th row after you did that first decrease row. Then again on the next 6th row. At that point you will have 36 stitches because you began with 42 and decreased 2 stitches on each of 3 rows, for 6 stitches gotten rid of. 42-6=36

So do the decrease as given in your “Next row” above, then P a row (1), K a row (2), P a row (3), K a row (4), P a row 5. The next knit row will be the 6th row, that is a right side row and your next decrease row, again, done exactly the same as the first one, leaving you 38 stitches. Then count off another 5 rows and on the 6th row, repeat the decrease row again to get you to the 36.

Then work another 7 rows over the 36 stitches making no decreases or anything, just keeping it in stockinette.