Question

Is this a sequence -18,9,-3

Answer 1

Yes, dividing by -2

Answer 2

No it is not!
a sequence has to follow a pattern
sequence is arithmetic when there a common different between all the terms
like 8,6,4,2,0,-2,-4,-6
you see that the common different is -2 each time we subtract 2
the sequence is geometric when there is a common ratio denoted r that if we multiply it by the previous term we get the next term
here an example
1,1/2, 1/4, 1/8...... each time we are multiplying by 1/2 to get the next term
hence r=1/2
and so on...