Final answer:
To guard against glare from the headlights of a vehicle behind you, you should use a day/night mirror and adjust it before you drive.
Step-by-step explanation:
To guard against glare from the headlights of a vehicle behind you, you must use a day/night mirror and adjust it before you drive. A day/night mirror is designed to reduce glare by dimming the reflection when it detects bright light. This helps to improve visibility and minimize distractions caused by glare from headlights.