Do you understand what the question is asking you to do? You know that (note the "3", not "2", in the numerator. I assume that was a typo). You should also know that a "geometric series" is a series of the form and that the sum of such a series is as long as |r|< 1 so that the series converges.
Now look at . That is not quite in the form but can be put in that form. Divide both numerator and denominator by 27 and you get . Now do you see what "a" and "r" must be?