One week, Isabelle bought 2 bags of apples and 3 bags of oranges. The next week, she bought 1 bag of apples and 2 bags of oranges. If a represents the cost of each bag of apples and r represents the cost of each bag of oranges, which
expression could Isabelle use to calculate the difference between the amounts she spent each week?
3a-7
3a-57
(2a-3r)-(a-21)
O (2a+3r)-(a+21)