Answer
(a) $35cm$
(b) $36.9cm$
Work Step by Step
(a) We know that
$L=f_1+f_2$
We plug in the known values to obtain:
$L=30.0+5.0=35cm$
(b) We can find the distance between the lenses as
$d_i=(\frac{1}{f_1}-\frac{1}{d_{\circ}})^{-1}$
We plug in the known values to obtain:
$d_i=(\frac{1}{30}-\frac{1}{500})^{-1}=31.9cm$
Now $L=5.0+31.9=36.9cm$
