Answer:
$313.41
Explanation:
The formula for calculating the monthly payment on a loan for a certain number of years when compounded monthly is given by:

Here
PV = loan amount
i = interest rate per month in decimal
n = number of months
We have the value of the car as $15,700. However Hunan paid 15% down so the actual loan amount is 85% of the value = 15,700 x 0.85 = $13,345.00
The annual interest rate is 6% = 6/100 = 0.06 in decimal
Monthly interest rate = 0.06/12 = 0.005
Number of months = 4 years x 12 = 48 months
Plugging in these values into the formula gives

So the monthly payment is $313.41
Just use a calculator which can compute the monthly payment given all the values. There are plenty on line