Answer
a) virtual
b) $-45cm$
c) concave
Work Step by Step
(a) We know that the image is virtual as the image is on the same side of the object and it is viewed through the lenses and can't be captured on the screen.
(b) We know that
$d_i=-0.67d_{\circ}=-0.67\times 23cm=-15.41cm$
$f=\frac{d_{\circ}d_i}{d_{\circ}+d_i}$
We plug in the known values to obtain:
$f=\frac{23(-15.41)}{23-15.41}=-45cm$
(c) We know that the lens used is concave because the focal length is negative.
