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." (Should be "whose" instead of "who's" because it refers to possession.)
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"Don't forget to pack your coat and hat, because its going to be snowing in Minnesota all week." (Should be "it's" instead of "its" because "it's" is the contraction for "it is.")