How do you secure a car with a manual gearbox against rolling away on a downhill slope?

You answered correctly. When parking a car with a manual gearbox on a downhill slope, you should always apply the parking brake and engage either first gear or reverse gear. This combination ensures that, even if the parking brake fails, the engaged gear will help prevent the car from rolling away. Never leave the gear lever in neutral on a slope, since that removes any resistance from the engine, increasing the risk that the car could move unintentionally. This way, you maximize safety according to German traffic rules.