Look at this series: 7, 10, 8, 11, 9, 12, … What number should come next?

7+3=10

10-2=8

8+3=11

11-2=9

9+3=12

The series is following alternating number addition and subtraction. First, 3 is added, then, 2 is subtracted, then 3 is added, and so on.