Question

How  do  you  find  the  focal  length  of  a  concave  mirror?

Answer 1

if we know the image distance and object distance then we can find focal length

$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$

where u = object distance
v = image distance

Answer 2

1 Hold a concave mirror and direct its reflecting surface towards the sun
2 direct the sunlight on a sheet of paper to obtain a sharp image
3 distance between mirror and sheet of paper gives approximate focal length of mirror