The average life expectancy in the United States, L, has been rising steadily over the past few decades, as shown in the table below. Let t be the number of years since 1920, (so t=0 represents the year 1920). Year 1920 1930 1940 1950 1960 1970 1980 1990 Life expectancy 53.7 56.1 59.7 62.2 66.6 69.2 71.8 74.1 (a) Find the equation of the regression line.