\(\frac{1}{f_{eq}}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{L}{f_1 f_2}\)

\(\frac{1}{f_{eq}}=\frac{1}{3}+\frac{1}{3}-\frac{2.8}{(2)(3)}\)

\(f_{eq}=\frac{9}{3.2}=2.8125cm\)

\(M=\frac{25}{f_{eq}}+1=\frac{25}{2.8125}+1=9.89\)