Estimating number of slaves imported before 1790

I have a statistics problem I am having a hard time figuring out how to model mathematically.

The 1790 US Census counted 697,681 slaves and 59,196 free Africans in the United States.

(A) assume importation began in 1620 and increased according to some unknown exponential function

(B) assume a rate of natural increase at 2.5% per annum

(C) assume all free Africans are manumitted slaves or their descendants and the rate of manumission is constant during the entire period

Given these assumptions, what is the function that would describe the annual number of slaves imported during that time (1620-1790)?

I have a statistics problem I am having a hard time figuring out how to model mathematically.

The 1790 US Census counted 697,681 slaves and 59,196 free Africans in the United States.

(A) assume importation began in 1620 and increased according to some unknown exponential function

(B) assume a rate of natural increase at 2.5% per annum

(C) assume all free Africans are manumitted slaves or their descendants and the rate of manumission is constant during the entire period

Given these assumptions, what is the function that would describe the annual number of slaves imported during that time (1620-1790)?