Final answer:
Eliza's math book weighs 4 pounds, determined by subtracting the backpack's weight without the book from its weight with the book.
Step-by-step explanation:
Eliza's backpack weighs 18 pounds with her math book and 14 pounds without it. To find out how much Eliza's math book weighs, we simply subtract the weight of the backpack without the book from the weight of the backpack with the book.
Weight with book - Weight without book = Weight of book
18 pounds - 14 pounds = 4 pounds
Therefore, the weight of Eliza's math book is 4 pounds.