The scale factor in this polygon comparison can easily be found as 1.5 because when we divide the length of for example side PS (3 units) by the size of side AD (its pre-image which measures 2 units) we get:
3/2 = 1.5
We could try also the same with any other of the sides in image and pre-image, like PQ comared to AB. PQ has a length of 1.5 units, while AB has a length of 1 unit. Then:
PQ/AB = 1.5/1 = 1.5
