Answer:
6
Explanation:
Why don't we try writing out a couple of terms of this sequence? 6(1)+3, or 9; 6(2)+3, or 15; 6(3)+3, or 21; and so on. The jump from 9 to 15 is 6; from 15 to 21 is also 6; and so on. The common difference is thus 6; each new term will be 6 more than the previous term.