Books Purchase Problem

A bookstore is having a sale where all novels are priced at $12 each, while all non-fiction books are priced at $15 each. Let x represent the number of novels purchased, and y represent the number of non-fiction books purchased.

To find the total cost of the books purchased, the following equation can be used:

$$ 12x + 15y = C $$

where C is the total cost in dollars. If Alice buys 3 novels and the total cost is $81, how many non-fiction books did she purchase?