Hard Math Problem - Find Area of the Green Square

Great right triangle problem. Easy to make wood puzzle, laser cut on a Universal laser. Fun for a student to play with and figure out. Link to plan:
drive.google.c...
See all curriculum at Measurement toolbox.com
sites.google.c... Some of my favorite tools: www.amazon.com...

[@XLEZZY_REUPLOADS]

I have no idea how to do this.