f(x) = 4(3)x and f(5) = 972.
972 represents the population of the rabbits after five years.
5 in f(5) represents the year.
4 in f(x) = 4(3)x represents the starting population of the rabbits
3 in f(x) = 4(3)x represents the rate the population increases
x in f(x) = 4(3)x represents the number of years that have passed.