how can we determine the focl length of plane mirror?

The radius of curvature of a mirror is twice the focal length. In a plane mirror, the radius of curvature is infinitely long, so the focus will be at infinity. Another way to say it is that a plane mirror has no curvature, and as curvature becomes increasingly small, focal length becomes increasingly long. At a curvature of zero, focal length becomes infinite. The rays reflected from the plane mirror surface do not meet and hence the image formed is virtual.