Use the information .." In 1986, the population was 37200 people. In 1994, it was 42000 people." to find the growth rate in this period, i.e.,
(42,000 37,200)/8 = 4800/8=600
where 8 is the number of years from 1986 to 1994. This number, 600, is your m in the equation of a line like y=mx+b.
So, how do you find b?
We now that in 1986 the population is 37,200 and that x = 19861984 =2. Hence:
37,200 = 600*2+ b
from which we find b = 36,000. In this way we have found the equation we were after, i.e., :
y = 600*x + 36,000
where x is the number of years past 1984. So x=1 represents 1985, x=2 represents 1986, and so on.
Note that for x=10, i.e. in 1994, your linear equation gives a population of 600*10+36,000 = 42,000 which is exactly the number you were given for the population in 1994.
7/30/2013

Maurizio T.