Roberto's employer offers a sliding paid vacation. When he started work, he was given three paid days of vacation. For each six-month period he stays at the job, his vacation is increased by two days.
a. Let x represent the number of 6-month periods worked and y represent the total number of paid vacation days. Write an equation that models the relationship between these two variables.
b. How much vacation time will he have earned after working for 4.4 years?