Brianna needs 12 * 2 = <<12*2=24>>24 books to get her through the year.
She will borrow 8 - 2 = <<8-2=6>>6 new books from the library.
With the books she was given, the books she bought, and the library books, she will have 6 + 8 + 6 = <<6+8+6=20>>20 new books to read this year.
Thus, she will need to reread 24 - 20 = <<24-20=4>>4 of her old books this year.
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