Answer
7.20 m/s.
Work Step by Step
For a plane mirror, your image is the same distance behind the mirror as you are in front of the mirror.
The velocity of your image relative to the mirror is equal in magnitude and opposite in direction to your own velocity. In other words, both you and your image are moving away from the mirror surface at 3.60 m/s, though in opposite directions.
Your image is moving at 7.20 m/s relative to you.
