Answer:
-10
Explanation:
So it skipped labeling a marker as a number between 10 and 2. The difference between 10 and 2 is 10-2=8.
Half of 8 is 4. So it is going down by 4 each time.
So start at the marker called 10.
The next marker would be 10-4=6.
The next labeled marked is 2 which would do get 2 from doing 6-4=2.
The next one below 2 would be 2-4=-2.
The next one below that one would be -2-4=-6.
The ? would be -6-4=-10.
I also provided a drawing.