The two sentences that include errors in the use of possessives or contractions that should be corrected 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" instead of "its" (it is going to be).
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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" instead of "who's" (whose refers to possession).