Answer
$ f=\frac{md_{\circ}}{m-1}$
Work Step by Step
We know that
$\frac{1}{f}=\frac{1}{d_{\circ}}+\frac{1}{d_i}$
$\implies \frac{1}{f}=\frac{1}{d_{\circ}}-\frac{1}{md_{\circ}}$
$\implies \frac{1}{f}=\frac{1}{d_{\circ}}(1-\frac{1}{m})$
This simplifies to:
$\implies f=\frac{md_{\circ}}{m-1}$