Answer:The number of books that he had before last week is 28
Explanation:
Let x represent the number of books that he had before last week.
Demetrious returned five library books last week. This means that the number of books left is x - 5
He returned three books this week. This means that the amount of books left is x - 5 - 3 = x - 8
He then checked out 8 more books. This means that the amount of books left is x-8-8 = x - 16
He now has 12 library books. This means that
x - 16 = 12
x = 12 + 16 = 28