The first clue is a recap on using the apostrophe to show plural possession. Remember the rules:
1. If the root word is made plural by adding an 'S' (e.g. singular = dog, plural = dogs) then you show plural possession by placing the apostrophe AFTER the S: e.g. dogs'
2. If the root word is plural itself (e.g. children, people, women...) then the apostrophe is placed BETWEEN the word and the S: e.g. children's