John and Paul want to go to the cinema, they know that the ticket costs $1 and each food box cost $3. If they spent $14, how many food boxes did each of them buy?
In this case since there is two people, we include that 2. 3x is representing the cost for buying x food boxes and 1 the cost for the ticket. 14 is the total amount of money spent.
Each of them buy 2 food boxes.