b, \(\Delta FON\sim\Delta FA'B'\)
\(\Rightarrow\frac{ON}{A'B'}=\frac{OF}{A'F}\Leftrightarrow\frac{A'B'}{A'F}=\frac{ON}{OF}=\frac{4}{6}\)
\(\Rightarrow A'B'=\frac{2}{3}A'F\)
\(\Delta AOB\sim\Delta A'OB'\)
\(\Rightarrow\frac{OA}{OA'}=\frac{AB}{A'B'}\Leftrightarrow\frac{OA'}{A'B'}=\frac{OA}{AB}=\frac{12}{4}\)
\(\Rightarrow A'B'=\frac{1}{3}OA'\)
\(\Rightarrow\frac{2}{3}A'F=\frac{1}{3}OA'\)
\(\Rightarrow\frac{2}{3}\left(OF-OA'\right)=\frac{1}{3}OA'\)
=> OA' = 4cm => A'B' = 4/3 cm