Answer
a) $l=14cm$
b) $M=130X$
Work Step by Step
a) $d_i=\frac{f_od_o}{d_o-f_o}=\frac{0.740cm\times0.790cm}{0.790cm-0.740cm}=11.69cm$
$l=d_i+f_e=11.69cm+2.80cm=14.49cm$
b) $M=\frac{N}{f_e}\bigg(\frac{l-f_e}{d_o}\bigg)=\frac{25.0cm}{2.80cm}\bigg(\frac{14.49cm-2.80cm}{0.790cm}\bigg)=130X$