in 1990, the life expectency of males in a certain counrty was 66.6 years. in 1994, it was 68.9 years. Let E represent the life expectency in year t and let t represent the number of years since 1990.
E = 66.6 + 0.575 t
That formula indicates E = 66.6 years in 1990 and increases 0.575 years per year after that.