If the credit spread is 90 basis points, then the yield to maturity (YTM) on your debt should be the YTM on similar treasuries plus 0.9%. The fact that new 10-year treasuries are being issued at par with coupons of 4.5% means that with a coupon rate of 4.5%, these notes are selling for $100 per $100 face value. Thus their YTM is 4.5% and your debt’s YTM should be 4.5% + 0.9% = 5.4%. The cash flows on your bonds are $5 per year for every $100 face value, paid as $2.50 every 6 months. The 6-month rate corresponding to a 5.4% yield is 5.4%/2 = 2.7%. Armed with this information, you can use Eq.(6.3) to compute the price of your bonds.