Answer:
11. men's
12. airplanes'
13. companies'
14. sheep's
15. teeth's
16. brothers-in-law's
Step-by-step explanation:
Irregular plural nouns like men, sheep and teeth take 's possessive case. However, it is unnecessary to add 's to regular plural nouns that end in -s; so nouns like airplanes and companies can just take the apostrophe ' form when forming plural possessive case.