Answer
$12.0cm$
Work Step by Step
We can find the separation between the lenses as follows:
$d_{i1}=\frac{(f)(d_{\circ 1})}{d_{\circ 1}-f}$
$\implies d_{i1}=6cm$
and $d_{\circ 2}=x-6.0cm$
$d_{i2}=\frac{d_{\circ 1}d_{\circ 2}}{d_{i1}}$
$d_{i2}=\frac{(12cm)(d_{\circ 2})}{6cm}=2d_{\circ 2}$
Now $\frac{1}{f}=\frac{1.50}{x-6.0cm}$
$1.50f=x-6.0cm$
This simplifies to:
$x=12.0cm$
