Donald loves cute animals, so he goes to the animal shelter and adopts a fluffy rabbit and 2 cats. He soon realizes that the rabbit he adopted is expecting a litter of baby bunnies! How exciting! His house soon becomes a menagerie since the number of rabbits increases 4-fold every month. The cats, however, are all boys and so (thankfully!) they are not multiplying. By the time there are 18 cute animals, Donald decides he has had enough and starts to advertise the rabbits for adoption.

Let x=0 represent the time since Donald adopted the cute animals.
Enter an equation to model the total number of cute animals over time.