The price of the computer is 4 285 dollars which you borrowed for an installment plan.
The amount you borrowed is 4 285 dollars.
Interests = 513.26 dollars for 36 months
=> 36 months / 12 = 3 years
=> 4 285 + 513.26 = 4 798.26 dollars
=> 4 798.26 dollars / 3 years = 1599.42 dollars per year
=>599.42 dollars / 12 months 133.29 dollars per month