If the empty suitcase weighs 5.7 pounds and each book weighs 2.4 pounds, the total weight can be written as:

where n is the number of books.
If the total weight has to be less or equal than 50, we can write:

As n<=18.45, we know that Jessie can take up to 18 books, as the number of books is an integer.
We can check if the total weight is under 50 as:

The inequality is satisfied, and if we add one more book the weight goes to 48.9+2.4=51.3, which is over the limit.
Answer: 18 books.