Answer:
a) Total Interest Paid in 24 months is $1680
b) Total Cost of the car is $12180
c) Monthly Payment is $420
d) Annual Percentage Rate  is 10.47%
Explanation:
(a) Loan Amount = $8400
Interest Rate = 10%
Monthly Interest = 8400 x (10%/12)
              = $70
Total Interest Paid in 24 months = 24 x 70
                           = $1680
(b) Total Cost of the car = Loan Amount + Interest Paid + Down payment
                    = 8400 + 1680 + 2100
                    = $12180
(c) Monthly Principal Payment = 8400/24
                         = $350
Monthly Payment = Monthly Interest Payment + Monthly Principal Payment
               = 70 + 35
               = $420
(d) Annual Percentage Rate = (1+ 0.10/12)12 - 1
                       = 0.1047
                        = 10.47%