Hi and welcome.
The buttonhole will be where you bind off 3 stitches. Knit to the end of the row. Turn. Work to the point where you bound off 3 stitches, then cast on 3 stitches, work to the end of the row. Your buttonhole will be there. If you check the Free Videos I'm pretty sure there is a video about buttonholes. HTH
I should never overestimate my basic arithmetic skills.