12
First figure out of it is a arithmetic or geometric sequence, in this case it is a geometric sequence because you have to multiply, not add to find the next number.
Divide one number, I'll pick 324 by the number before it, 108, you get three.
Now divide 36 by 3, you get 12.
To check, multiply 12 by 3, you get 36, multiply that by 3, you get 108, and so on and so forth.
