Help 7TH GRADE math I give out good rating and BRAINIEST!

Answer:
12.6
Step-by-step explanation:
If 21 divided by 5 is 4.2, you should multiply 3 by 4.2 you'll then get 12.6. to double check, do 5 divided by 3, which is 1.67, and 21 divided by 12.6 is also 1.67. so side OP is 12.6! :)
Answer:
i don't know sorrryyy
Step-by-step explanation:
im very sorry because im sor busy