Question

What is the common ratio of the sequence?

-2, 6, -18, 54,...

Answer 1

Have you considered calculating the successive ratios?

6 / (-2) = ??
-18 / 6 = ??
54 / (-18) = ??
etc...

Answer 2

nah not yet.

Answer 3

if we are talking about sequences and ratio, you have a geometric sequence
the standard form of a geometric sequence is

a, ar, ar^2, ar^3, ar^4.... ar^(n-1)

so to get the common ratio, you must do an+1/an

so let n=1
an=a1=a
an+1=a2=ar

ar/a=r

let n=2
an=a2=ar
an+1=a3=ar^2

ar^2/ar=r

so in your sequence

n=1, an=a1=-2
an+1=a2=6

6/-2=-3

n=2, an=a2=6
an+1=a3=-18

-18/6=-3

do this for all terms of your sequence to make sure they are the same r, ratio, throughout, hence this is your common ratio