Focal length of a silvered lens

Описание: Totally confused while finding the sign convention of a silvered plano convex lens. I know that equivalent power of such a system can be found by adding the power of the mirror and twice the power of the lens.cBut say in this example in the picture below the equivalent focal length has been considered negative. WHY? Isn't the focal length of a plano convex lens positive if the object distance is taken to be negative? Obviously power of a mirror is 0 so it can't affect the signs!