Final answer:
When towing a large trailer that obstructs your inside rear-view mirror, it is required to have outside rear-view mirrors installed on both sides of your vehicle.
Step-by-step explanation:
When towing a trailer large enough to obstruct your inside rear-view mirror, you are required to have outside rear-view mirrors installed on both sides of your vehicle.
This is to ensure that you have adequate visibility to the rear when driving.