This is a simple alternating subtraction series, which subtracts 2, then 5.
Rule = (first number - 2) (second number - 5) (third number - 2) (fourth number - 5)...
Now The series is 31, 29, 24, 22, 17, . . .
(31 - 2 = 29), (29 - 5 = 24), (24 - 2 = 22), (22 - 5 = 17), (17 - 2 = 15)...
So the answer is a = 15.