For lens A
\(\frac{h_i}{h_0}=\frac{-(-v)}{u}\)
\(\frac{2}{6}=\frac{+v}{-20}\)
\(v=-\frac{20}{3}\,cm\)
u = - 20 cm
\(v=\frac{-20}{3}\,cm\)
\(\frac{1}{f}=\frac{-3}{20}+\frac{1}{20}=\frac{-20}{20}=\frac{-1}{10}\)
f = - 10 cm
\(P=\frac{100}{-10}=-10\,D\)
For lens B
\(\frac{2}{6}=\frac{-v}{-20}\)
\(v=+\frac{20}{3}\,cm\)
\(\frac{1}{f}=\frac{3}{20}+\frac{1}{20}=\frac{4}{20}\)
\(f=\frac{20}{4}=5\,cm\)
\(P=\frac{100}{5}=20\,D\)