Answer:
18, 21
Step-by-step explanation:
By finding the difference between each term, you can find the pattern. 9-6=3, 12-9=3, 15-12=3, so we know that each term is 3 more than the last.
Therefore, by following the pattern of +3, the next term is 15+3=18 and the one after that is 18+3 = 21
