9) Pablo wants to save money for a new piano. He opened a savings account and he noticed that 4 months after opening it he had $160 and 9 months after opening it he had $235. If the amount of money in his account follows the model of a linear function, find the following:

a. (4 points) How much is his savings account increasing each month?

b. (4 points) What was his initial investment amount?