Take the two prices (54.00 and 21.99) and add them together
this equals 75.99
then multiply this by .15 (15%)
this equals 64.5915
then take this and multiply it by .075 (7.5%)
this equals 4.8443625
then add 64.5915 (price after coupon) with 4.8443625 (the sales tax on the price after the coupon)
this equals 69.4358625
So in total, he paid $69.44 (rounded)