Since this is a linear (non-exponential) population problem you can just use the standard y=mx+b form of an equation. Where m = (change in population/change in years)
The numbers you were provided state that over the course of 7 years (1998-1991) the population increased by 420 people (4130-3710). So, (420/7) = 60 = m. Assuming that the growth rate for 1990 is the same as 1991. then you would have a starting population of (3710-60) or 3650, that would be your "b" value since at t=0 P(t) = 3650. This yields a final equation of P(t) = 60t +3650. Check the answer at t=1 and you get the population during 1991: 3710.