Question 1091386
Since the amount paid of $15.48 is  after using the coupon, the value of the coupon must be added to this amount to get the original price:

Let y = original price = $15.48 + $5.16
        
Solving just requires adding the two numbers on the right hand side:
   {{{ highlight(  matrix(1,4,   y, "=", "$", 20.64)) }}}