I used substitution and rearranged the second equation to y=-2x+1. Then I replaced the y in the first equation with -2x+1. So, I got 3x-5(-2x+1)=21. When you solve that you get 13x=26 or x=2. Then you replace x with 2 in either equation and end up with y=-3.
The nearest whole number is 17. when you divide 3960 by 242, you get 16.363636.... but, he wouldn't have completely paid it off at 16 months, so it would be fully paid at 17 months