A point object 'O' is at the centre of curvature of a concave mirror. The mirror starts to move at a speed u, in a direction perpendicular to the principal axis. Then, the initial velocity of the image is

Image velocity (w.r.t. mirror) = m × object velocity (w.r.t. mirror)Here m = -1. The negative sign shows that  the image is moving opposite to that of the mirror. As the mirror is also moving with a speed u, therefore the total speed of the image will be u+u=2u.