First let y represent the life expectancy and let x 
represent the number of years since 1990.  Then later 
we'll replace y by E(t) and x by t.

Then when x=0, then y=72.4, and
     when x=4, then y=74.9

So the problem now becomes:

Find the equation of the line which contains the points
(0,72.4) and (4,74.9).

Use the slope formula:

  

Use the point-slope form:






Now replace  by  and  by 



To find the life expectancy after 18 years from 1990.



Since 18 years from 1990 is this year 2008, then if that
formula is a correct predictor, then the life expectancy
at present should be  years.

Maybe it'll be over 100 years by the time you get old!

Edwin