I believe you would start by noting the slope intercept form of an equation which is: y=mx+b, now proceed to find the slope. The slope can be found by taking delta y divided by delta x, or change in y over change in x. In this case you will make the years the y, and the change in population x. So calculate the change in years divided by change in population to get your slope, m. Now plug back in the equation, y=mx+b, and that should be it. Hope this helps!