Answer
$-30.0cm$
Work Step by Step
We know that
$d_{i1}=\frac{d_{\circ1}f}{d_{\circ1}-f_1}$
We plug in the known values to obtain:
$d_{i1}=\frac{(30.0cm)(20.0cm)}{30.0cm-20.0cm}$
$d_{i1}=60.0cm$
Now we can find the required focal length
$\frac{1}{f_2}=\frac{1}{d_{\circ2}}+\frac{1}{di2}$
We plug in the known values to obtain:
$\frac{1}{f_2}=\frac{1}{-20.0cm}+\frac{1}{60.0cm}$
$\implies f_2=-30.0cm$
