Answer
(a) $-84cm$
(b) $42cm$
Work Step by Step
(a) We can find the image distance as
$d_i=-md_{\circ}$
We plug in the known values to obtain:
$d_i=-84cm$
(b) We can find the focal length as
$f=\frac{1}{d_{\circ}}+\frac{1}{d_i}$
We plug in the known values to obtain:
$f=(\frac{1}{28}+\frac{1}{-84})^{-1}$
$f=42cm$
