...which you can arrive at with one calculation - namely the ratio of the delta-x coordinates of the two polygons
...we're just concerned with magnitudes here, so we'll ignore the negative sign in the delta x for the second polygon. The magnitude of this difference is 4.
The ratio of these is the scale factor.
This is the same ratio as that for the corresponding sides of the 2 polygons.
I.e, the length of AB:
length of A'B' =
...and 8.5/17 = 0.5, your scale factor. Doing it the second way, using the pythagorean theorem, is just a few more calculations.