Answer:
5 books.
Explanation:
She can carry 18 pounds in total. She already has her lunch and her gym clothes inside her bag, which take up 1 + 2 = 3 pounds. Since 3 pounds are taken, that leaves 18 - 3 = 15 pounds for her books. Each book is 3 pounds, so she can carry 15/3 = 5 books in her backpack.