A tool rental shop can rent 30 floor sanders per day at a daily rate of $12 per sander. For each additional $2 a day charged per sander, one fewer sander is rented. If the store wants to maximize its revenue, what should it charge to rent a sander for one day? (Give your answer to the nearest dollar)

i used excel to find all the possible revenue by multiplying different daily rates and number of sanders rented to find the answer.

however, is there an equation? i cannot seem to set one up.