Take this for example:
A curious guys quest for answers.
The guys part, is it guys’ guy’s, or guys? And why?
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I assume that you are referring to one curious guy, and in that case you would need guy’s for the possessive form:
A curious guy’s quest for answers
For the possessive form of the singular noun “guy”, add 's.
grammar.ccc.commnet.edu/grammar/possessives.htm
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