Answer:
You can not tell unless you have information about the relationship between the small and large boxes (for example, a large box is 2 times the weight of a small box) You can only tell that a small box and a large box's weight together is 34.5 kilograms.
Step-by-step explanation:
3L + 5S = 135
9L + 7S = 279
12L + 12S = 135 + 279
12(S+L) = 414
S+L = 34.5