If the number is a multiple of 3 but not of 6, the number must be odd. The only odd multiple of 3 between 17 and 25 is 21, meaning that this is the answer. Hope this helps!
I knew that the hypotenuse was 13 so it couldn’t be more than 13. I was guessing at numbers until I got to 5 then added 7. After I use Pythagorean theorem to check and I got 13.
