Answer
a) $f_1$
b) $d_i=1.20m$
Work Step by Step
(a) We should use a convex lens to project a real image onto the wall. Since the convex lens has a positive focal length, the lens having the focal length $f_1=+40.0cm$ should be used.
(b) We know that
$\frac{1}{f}=\frac{1}{d_i}(1-m)$
This can be rearranged as:
$d_i=(1-m)f$
We plug in the known values to obtain:
$d_i=(1-(-2.00))(40.0cm)$
$d_i=120cm=1.20m$
