Final answer:
The desk exerts an upward force of 32 N on the book.
Step-by-step explanation:
The desk exerts an upward force of 32 N on the physics book. This is because, according to Newton's third law of motion, every action has an equal and opposite reaction. So, when the book exerts a downward force of 32 N on the desk, the desk exerts an equal and opposite upward force of 32 N on the book.