Samuel orders four dvds from an online music store. each dvd cost $9.99. he has a 20% discount code and sales tax is $6.75. what is the total cost of his order?

this is a review and i forgot what to do.

Number of DVDs * Cost of DVDs = Total Cost of DVDs
4 * 9.99 = 39.96

10% of 39.96 = 3.996 
3.996 * 2 = 20%
7.992 = 20%

Total Cost of DVDS - Discount = Cost
39.96 - 7.992 = 31.968

Cost + Sales Tax = Total Cost
31.968 + 7.75 = $39.718

The answer is $39.718 or $39.72

$14.74
you take 9.99 times 4 and get 39.96
you convert 20% to a decimal, which is 0.2
you then multiply it to 39.96
finally you add on the tax price of 6.75