To determine the scale factor, we can compare the distances between the points on the original line AB and the corresponding points on the dilated line A'B'.
The distance between A and B on the original line AB is 6-9 = -3 units.
The distance between A' and B' on the dilated line A'B' is 2-3 = -1 unit.
Therefore, the scale factor used in the dilation of line AB is equal to the ratio of the distances: (-1)/(-3) = 1/3.
So, the correct answer is: Start Fraction 1 over 3 End Fraction.