Explanation:
the money that Shelly will owe after buying x sandwiches and using the card will be:
if x < 4 then 15 - 4.5x <= 15 - 3*4.5 = 1.5 she will not owe anything plus she will receive change of $1.5.
if x=> 4 then 4.5x - 15 => 18 - 15 = 3 that is Shelly will owe 4.5x - 15 dollars.