Answer: The next two terms are 33 and 39
Explanation:
9,15,21,27
First determine how you get from one value to the other.
To get from 9 to 15 you add 6.
To get from 15 to 21 you add 6 to 15.
And to get from 21 to 27 you add 6 to 21
So to get to the next term add 6 and another 6 to the value you get.
27 + 6 = 33
33 + 6 = 39