Final answer:
The number of books Debbie has is three times the number Tiffany has. So if Tiffany has x books, Debbie has 3x books.
Step-by-step explanation:
The question involves a basic algebra concept where we are asked to express a relationship between the number of books Debbie has compared to Tiffany. If Tiffany has x books, and Debbie has three times the number of books that Tiffany has, we use multiplication to find the number of books Debbie has. Therefore, the expression for the number of books Debbie has would be 3x.