The common ratio refers to the number that multiplies the previous term to get the next term.
For me, -2 pops out as the answer. Let's test it out.
First, multiply -2 by 3.
3 * -2 = -6
Now we have our second term and it matches up with -6 in 3,-6,12,-24,48.
Now multiply it again by -2.
-6 * -2 = 12
Yup, we can say -2 is the answer to this question.