From question \(: v=15\, \mathrm{cm}, u=-10\,\mathrm{cm}\)
Radius of curvature, \(R=2\,\,f=?\)
Using mirror formula, \(\frac{1}{v}+\frac{1}{u}=\frac{1}{f}\)
\(\frac{1}{15}+\frac{1}{(-10)}=\frac{1}{f} \Rightarrow f=-30\, \mathrm{cm}\)
Therefore radius of curvature,
\(R=2\, \,f=-\,60\, \mathrm{cm}\)