Sir you have said that when the lens is silvered on the plane surface the focal length of the lens becomes equal to two times the focal length of the mirror. i didn't understand how [focal length of lens (f)= 2×fm] please explain this concept if possible with a diagram.

When the light enters the lens from air, it gets focussed at a distance equal to the focal length of the lens. Now, if the plane surface had not been silvered, then the light would get focussed at f.

But,  this light encounters the plane silvered surface. This will now act as a plane mirror for this light.

Now, we know that the plane mirror forms an image inside the mirror at a distance equal to the distance of the object from it.

So, basically we can see that the image will be at a distance equal to twice the focal length of the mirror.