The two sentences that include errors in the use of possessives or contractions are:
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"Don't forget to pack your coat and hat, because its going to be snowing in Minnesota all week." (It should be "it's" which is a contraction for "it is.")
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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." (It should be "whose" which is the possessive form used to indicate ownership.)