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

To prevent a two-axle truck from rolling down a steep slope, both engaging the parking brake and placing a wheel chock under a rear wheel are necessary. The parking brake secures the vehicle in place, while the wheel chock acts as an additional physical barrier to prevent movement. This combination is crucial as it adheres to safety regulations outlined in German traffic laws, which emphasize the importance of securing parked vehicles, especially on inclines. Additionally, while applying retarders can help slow down the vehicle, it is not sufficient alone for preventing rolling when parked; hence, it is not included in the necessary actions for securing the truck on a slope.