The answer is 15 days.
The first step you need to take is to subtract the amount of cards he has from the amount of cards he needs. So 334 (needs) - 214(has) = 120 cards he still needs. So he is collecting 8 cards a day. In order to find out how many days it will take to collect the 120 cards, we need to divide 120 by 8. 120 divided by 8 is 15. So it will take 15 days collecting 8 cards a day in order to reach the 120 cards he still needs to collect.