Find a formula for the general term a_n of the sequence:

{-3, 2, -4/3, 8/9, -16/27}

First of all, we need to put the first two numbers into fraction form:
{-3/1, 2/1, -4/3, 8/9, -16/27}
You can see there is a pattern to these numbers, and the pattern is that each number is multiplied by -2/3 to give the following number (-3*(-2/3) = 2; 2*(-2/3)=-4/3, and so on).
This means that this is a geometric sequence.