n1950, the population density of the United States was 42.6 people per sq mile and in 2000 it was 79.6 people per sq mile. Assuming the population density grew at a constant rate of change, find the function that gives the population density based on the number of years since 1950.

So far all I have gotten isy = .74xbut I know I need it iny =.74x + b.I just can't figure out the +b part!

Also, would x = the year?

Thank you!