Triangle A is the original triangle, and triangle A' is the image.
We need a pair of corresponding sides in the two triangles that are both labeled with lengths.
We see the side labeled 15.2 mm in triangle A', and we see the side labeled 3.8 mm in triangle A. These two sides are corresponding sides.
Divide the length of the side in the image by the length of the side in the original to find the scale factor.
(15.2 mm)/(3.8 mm) = 4
The scale factor is 4.
