Answer
f = 46 mm.
Work Step by Step
Find the image distance using the magnification equation. The image is inverted.
$$m=\frac{h_i}{h_o}=-\frac{d_i}{d_o}$$
$$d_i=-d_o\frac{h_i}{h_o}=-(65m) \frac{-24mm}{34m}=45.9mm$$
Now find the focal length from the lens equation.
$$\frac{1}{d_o}+\frac{1}{d_i}=\frac{1}{f}$$
$$f=\frac{d_od_i}{ d_o+d_i }=\frac{(65m)(0.0459m)}{ 65m+0.0459m }=0.0458m\approx 46mm$$
