The birth rate in a certain country in 1991 was 15.3 births per thousand population. In 2001, the birth rate was 14.29 births per thousand. a. Let x represent years after 1991 and y represent the birth rate. Assume that the relationship between x and y is linear over this period. Write a linear equation that relates y in terms of x. b. Use the linear equation from part (a) to estimate the birth rate in this country in the year 2021.