The first term of an arithmetic sequence is 12. The 14th term is -108 what is the common difference

Acarnival has a duck pond booth. you choose a rubber duck at random. the mark on the bottom of the duck tells you whether you won a small, medium, or large prize, or no prize at all. there are 65 ducks floating in the pond. there are 3 marked as large-prize winners, 13 ducks marked as medium-prize winners, and 21 ducks marked as small-prize winners. find the theoretical probability of winning a medium prize at the duck pond. express your answer as a decimal. if necessary, round your answer to the nearest thousandth. 21/65 1/21 1/12 1/5