Number Line Pattern Recognition: Identifying the Missing Value

Let's pay attention to the numbers on the axis from left to right:

$-6,-3,0,?,6,9,12,15$

We'll try to find the common denominator.

Note that if we use the operation 3+, we'll reach the sequence of numbers:

$-6+3=-3$

$-3+3=0$

$6+3=9$

$9+3=12$

$12+3=15$

Therefore, the question mark will be the following exercise:

$0+3=3$