A coupon subtracts $5.16 from the price p of a shirt. You pay $15.48 for the shirt after using the coupon.
p = 15.48 + 5.16 (Or p - 5.16 = 15.48)
p = 20.64
The original price was $20.64. The coupon discount was $5.16, leaving $15.48 to be paid for the shirt.