Let's break down the situation step-by-step:
1. You are 10.5 meters in front of the plane mirror
2. Your reflection in the mirror is also 10.5 meters away from the mirror
3. Since light beams travel from the object/reflection through the lens to focus at the sensor/film, the distance to focus on is the distance from the reflection to the lens (camera).
4. The reflection is 10.5 meters away from the mirror, so the distance to focus on is 10.5 meters.
Therefore, to focus your camera on your reflection in the mirror, you should set the focus to 10.5 meters.