Answer:
First, frank buys 3 packs on 3 different days, so in those first 3 days he buys:
3*3 = 9 packs.
Then on other 3 days he buys 4 packs each day, so he has:
3*4 = 12 packs.
In total he haves:
9 + 12 = 21 packs.
Now he divides the packs with his 2 brothers (So we divide 21 packs between 3 people)
Then each brother gets:
N = 21/3 = 7 packs each.