The length is 12, because if it's 9 inches wide it has two sides of 9 inches which is 18. Then subtract that from the intial value 42 to get 24. After, you divide 24 by 2 due to there being 2 sides of the box and 24 is the sum of both sides. Hopethis helps!
The best estimate would be 121/15 = 8.1
I not that good with math but I would say C.
The total cost would be $4,050. Hope this helps!