I'd usually just look at the shape and do simple math to figure out what the missing number is. The equation way is confusing but I know if I watch it enough I should get it. Thanks so much.
Multiply the ration by each side. Example: If the sides are 3, 4, 5 with a ratio of 2:1 or 2/1 - you would multiply each side by 2, to arrive at the new side lengths.