For these, you can find the appropriate formula by looking at the ratio of the first two terms. That will be the number in parentheses in the explicit formula. If there is no match, it is <em>not geometric</em>.
6/3 = 2 . . . an = 3(2)^(n-1)
-6/3 = -2 . . . an = 3(-2)^(n-1)
6/2 = 3 . . . an = 2(3)^(n-1)
5/2 = 2.5 . . . not geometric (arithmetic sequence: an = 2+3(n -1))
