Answer
See work below.
Work Step by Step
The lens equation shows that when an object is at infinity, the image is at the focal point.
Consider equation 23–9 with the image distance equal to the focal length.
$$m=-\frac{d_i}{d_o}=-\frac{f}{d_o}=(-\frac{1}{d_o})f=(constant)(f)$$
For an object at a fixed distance, this result proves that the magnification is proportional to the focal length.
