The two sentences that include errors in the use of possessives or contractions are:
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"We are not certain who's textbooks these are, but we want to find out so that we can return them."
- Correction: Replace "who's" with "whose" (the correct possessive form).
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"Don't forget to pack your coat and hat, because its going to be snowing in Minnesota all week."
- Correction: Replace "its" with "it's" (the contraction for "it is").