To figure out this pattern, let's see if there's a constant rule that this sequence follows. From 24 to 21, you're subtracting 3. Let's try n-3, and fill in the blanks as we go. 24 - 3 = 21. 21 - 3 = 18. 18 - 3 = 15. 15 - 3 = 12. 12 - 3 = 9. It seems that n-3 works as a rule, so your rule is: N-3. The missing blanks are: 18, 12, and 9. I hope this helps!
