For a concave mirror, the radius of curvature is twice the focal length of the mirror:
[tex]r=2f[/tex]
where f, for a concave mirror, is taken to be positive.
Re-arranging the formula we get:
[tex]f= \frac{r}{2} [/tex]
and since the radius of curvature of the mirror in the problem is 24 cm, the focal length is
[tex]f= \frac{24 cm}{2} = 12 cm [/tex]
