Answer:
-2
Explanation:
This is a geometric sequence. We need to find the common ratio
Take the second term and divide by the first
-18/54 = -1/3
Take the third term and divide by the second
-18/6 = -1/3
The common ratio is -1/3
The next term is the third term times the common ratio
6 * -1/3 = -2
The fourth term is -2