Algebra
posted by Jerry .
In 1992, the life expectancy of males in a certain country was 69.8 years. In 1999, it was 72.6 years. Let E represent life expectancy in year t, and let t represent the numbers of years since 1992.

OK. And then what ?
You certainly cannot expect E to increase 2.8 years per 7 years (0.4 years per year) indefinitely.
Past performance is no guarantee of future results, but it might work for a year or two.