Missing pattern 24,21, blank 15 blank ,blank

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!

The pattern would be n-3

Definitely