Watch and take Cornell Notes on the following videos:
VIDEO #1 -- Solving for missing side lengths using proportions
https://www.youtube.com/watch?v=XLwmsUK1aJA
VIDEO #2 -- LEARN ZILLION -- Solving for missing side lengths using proportions and scale factors
https://learnzillion.com/lesson_plans/8714-find-a-length-by-using-a-scale-factor-and-a-proportion