Answer: It represents the fact that the base price of $7 is being subtracted from the amount which is x
Step-by-step explanation: They state here that the teacher needs 9 boxes and the base price is $7. but stored are having discounts so they need to accommodate for that price, they call it x. They also state that the whole amount that they paid was 9(7-x). The 9 is for the 9 boxes, the 7 is for the $7 base price, and finally the x is the discount from ever store. HOPE THIS HELPS :)
