NI would assume that the rabbits added are reproducing each month as well I just cannot develop a proper formula
When in doubt as to how to develop a modeling formula, it is frequently useful to show one's work explicitly.
At the start (at step 0), the population is 2 000.
At the one-month mark, the population is regarded as having grown by three percent: 1.03 * 2 000.
Also, twenty more units are added to the population: 1.03 * 2 000 + 20
At the two-month mark, the population is regarded as having grown by three percent: 1.03(1.03 * 2 000 + 20)
Also, twenty more units are added to the population: 1.03(1.03 * 2 000 + 20) + 20
Simplifying yields: 1.03
^{2} * 2 000 + 1.03 * 20 + 20
Follow the pattern for the three-month mark. Simplify. And again for the four-month mark. Continue until you are able to extract a generalized formula for the n-th-month mark.