The coupons are paid on the 1st of June and the 1st of December. You know this because there is always a coupon on the redemption date and can therefore work backwards. (Coupons are not necessarily paid on valuation date). So draw a timeline with this information and see if you can see where those n figures come from. If this still doesn't help just comment below and I'll explain it fully.
The 0.5 for the interest rate, remember that the annuity formula has to have the interest rate that relates to the time period of payments. ie since the coupons are paid every 6 months you need an effective 6 months (0.5 of a year) interest rate.