13. Bill bikes 4 km to school. After 5 min he is 3.2 km from the school. The graph of the function that mod- els Bill's distance to school is shown in red. Because of hills and traffic conditions along the way, Bill's speed varies. Nevertheless, a linear function, whose graph is shown in blue, can also be used to model his distance as a function of his time spent bicycling. a. Give an equation of the linear function. b. Use the equation to find approximately how long it takes Bill to bicycle to school​