The velocity of the image with respect to the observer will be $12 \mathrm{~m} / \mathrm{s}$. This is because the velocity of the image with respect to the mirror is the same in magnitude as the velocity of the object (the observer) with respect to the mirror, but in the opposite direction. So, if an observer is walking away from the plane mirror with a speed of 6 $\mathrm{m} / \mathrm{s}$, the velocity of the image with respect to the observer will be the sum of the observer's velocity and the image's velocity, which is $6 \mathrm{~m} / \mathrm{s}+6 \mathrm{~m} / \mathrm{s}=12 \mathrm{~m} / \mathrm{s}$