The series follows a pattern of increasing differences between consecutive terms. The difference between 31 and 43 is 12. The difference between 21 and 31 is 10. The difference between the missing term and 21 should be 8, and the difference between 7 and the missing term should be 6. Therefore, the missing term is 7 + 6 = 13. Checking the subsequent difference, 13 + 8 = 21, which fits the pattern.