In 1940 the life expectancy at birth in the general public was 62.9 years. by 1980, it had risen to 73.7 years. assuming a linear relation, which is the best prediction of life expectancy in the year 2000?

79.1 life expectancy in the year 2000
1940: 62.9
1980: 73.7
From 1940 to 1980, that was 40 years. It rose 10.8 years for life expectancy in those 40 years, therefore 0.27 life expectancy years per year. (10.8 divided by 40)
From 1980 to 2000, that is 20 years, so 20 multiplied by 0.27= 5.4 (from 1980 to 2000 the life expectancy rose 5.4 years) 
73.7 for 1980, plus 5.4 equals 79.1
Answer is 79.1 years of life expectancy in the year 2000

Abit confused about what you mean, but i think you might be referring to slope intercept form: y = mx + bin that case b, or the y intercept is already given.(0,-1)find the slope by using the formulay2 - y1/x2 - x1the final slope will be 4/2, equivalent to 2.now your equation should look like thisy = 4/2x -1sorry if this didn’t you out!