Answer:
f =-20 cm
Step-by-step explanation:
Given that,
The magnification of a spherical mirror, m = -1
The image distance, v = 40 cm (for negative magnification)
The magnification of a concave mirror is negative. The mirror showing -1 magnification is a concave mirror.
Let f be the focal length of the mirror. We know that,

Object distance, u = -40 cm
Using mirror's formula i.e.

So, the focal length of the mirror is 20 cm.