First off, you have to start of with a proportion. They will be two fractions and one of the fractions will have a number over x or x over a number. Then you multiply the top number of one of the fractions to the bottom number of the other fraction. Then you divide it by the number that is left, which is the number you didn't multiply anything with, and you would get your scale factor.
You multiply the two similar side lengths and divide it by the other similar side.
Measure the corresponding sides and if they share a ratio, the ratio is the answer