The scale factor is what u multiply the original image by to find the new image.
Now look at original image AD....it equals 3 units.....now look at new image A'D'....it equals 6 units.....so what do u have to multiply 3 by to equal 6 ? That would be 2.....because 2 * 3 = 6. Therefore, ur scale factor is 2.
u can try that on all ur points...and u will find that when multiplying the original image by 2, u get the new image.
answer : scale factor of 2