That look like an obvious decreasing trend. From the first and last point we get a slope of (9-31)/(92-26) = -1/3 and a y intercept of 31 - (-1/3)26 = 39.67.
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.
