Answer
(a) $d = f_1+f_2$
(b) $w_2 = \frac{f_2}{f_1}\cdot w_1$
Work Step by Step
(a) Parallel beams travel through the first lens, meet at the focus point after travelling a horizontal distance of $f_1$, travel a horizontal distance of $f_2$, then exit the second lens as parallel beams.
Therefore, $d = f_1+f_2$
(b) We can use symmetry to find $w_2$:
$\frac{w_2}{w_1} = \frac{f_2}{f_1}$
$w_2 = \frac{f_2}{f_1}\cdot w_1$
