In year 2000, the population was 15,000.
In year 2001, the population was 16,250.
In year 2002, the population was 17,500.
The population growth from year 2000 to year 2001 was (16,250-15,000) = 1,250.
The population growth from year 2001 to year 2002 was (17,500-16,250) = 1,250.
So the slope of the line would be m = 1250.
And y-intercept would be the initial population i.e. b = 15,000.
So the equation of line is y = 1250x + 15000.
Hence, n years after 2000, the population would be P = 1250n + 15000.