Answer:
The formula for the sequence is: 
Step-by-step explanation:
The general term of a geometric sequence is given by:

In which
is the first term and r is the common ratio between the terms, that is, the division between them.
What is the formula for the sequence 2, -6, 18, -54, ...?
First term is 2, so 
Common ratio is given by:

So the sequence is given by:
