Need help with Linear functions

How am I suppose to solve this problem? By creating a formula for it?

If so this is what I got so far; by taking the years 1980-1990, I tried to find the average rate of change of the population, 17.2 - 14.9/1990 - 1980, which came out to be 0.23. After that I tried writing up a formula for it by using y = mx + b. So it would be P = 0.23x + 17.2. After that in order to find what is the population in 1988 do I just put in an 8 in that equation?

P = 0.23(8) + 17.2 = 19.04?

Here is the question:

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