(a)\(L=f_{obj}+f_{oc}=12-4=8cm\)

\(M=\frac{12}{4}=3\)

(b)\(\frac{1}{s_{oc}}+\frac{1}{-30}=\frac{1}{-4}\)

\(s_{oc}=-\frac{60}{13}cm\)

\(L=12-\frac{60}{13}=7.83cm\)

\(M=\frac{12}{\frac{60}{13}}=2.6\)