If the interest rate for a 15-year fixed rate mortgage is 5.00%, then the corresponding 30-year fixed rate is 6.39%. If the 15-year rate decreases to 4.00%, then the 30-year rate decreases to 4.99%.

a. Assume that the relationship between the 15-year fixed rate (%) and the 30-year fixed rate (%) is linear. Find the equation of the line that describes this relationship. Write your answer in slope-intercept form. Let x be the 15-year fixed rate and the y be the 30-year fixed rate.

b. Use your equation from part a to estimate the 30-year fixed rate if the 15-year fixed rate is 4.5%.