To represent the inequality that Mari wants to spend at most $15 on cheese and crackers for the party, we can set up the following inequality:
6 + 3b ≤ 15
In this inequality, 6 represents the cost of the cheese and 3b represents the cost of the crackers (where b is the number of boxes of crackers Mari plans to buy). The sum of these costs should be less than or equal to $15, hence the inequality.