Synopticom Inc. plans a bond issue for the near future and wants to estimate its current cost of debt capital. After talking with the firms investment banker, the firms chief financial officer has determined that a 20-year maturity bond with a $1,000 face value and 8% coupon (paying 8% x $1,000 = $80 per year in interest) can be sold to investors for net proceeds of $908.32. If the Synopticom tax rate is 34%, what is the after-tax cost od debt financing to the firm?

My textbook says N=20 PV=-850 PMT=80 FV=1,000 to get R=9.73 I am confused where the 850 came from. Please explain, thank you.