What must you do to prevent a two-axle truck from rolling down a steep slope?

You are correct. To prevent a two-axle truck from rolling down a steep slope, you must engage the parking brake and also place a wheel chock under a rear wheel. The parking brake alone helps secure the vehicle, but using a wheel chock provides extra safety in case the brake fails or slips, especially on a steep grade. These two steps together make sure the truck stays safely in place. Using only a retarder is not enough, as retarders are designed to assist with braking while driving, not to keep a parked vehicle stationary. Always use both the parking brake and a wheel chock on steep slopes for maximum safety.