Complete the Arithmetic Sequence: Finding Missing Numbers After 15

Let's look at the numbers from left to right:

$-3,3,9,15$

We notice that the common operation is:

$+6$

$-3+6=3$

$3+6=9$

$9+6=15$

Therefore, the next sequence will be:

$15+6=21$

$21+6=27$