To finish the top triangle there's one more set of decreases that needs to be worked. Simply purl 2 together and turn, then slip, slip knit and turn. One stitch remains.
Leave this stitch on the right hand needle and pick up your 7 stitches for the next triangle you need to work.
For triangles of different sizes the concept is the same, decreasing at the end of each side until you are down to just one stitch.