Your function is not defined at x=0 and x=4 (you have vertical asymptotes there). So, the denominator must be zero at x=0 and x=4. This is the case for
So, the function with that denominator is graphed.
The $2,000 upfront payment by Hank means that the relationship between the total amount paid and the number of months elapsed is NOT linear. On the other hand, the girl's zero down payment indicates a linear, proportional relationship.