Here, we convert the focal length of the lens and the size of the slide to meters. The focal length $f$ is $12 \mathrm{~cm} = 0.12 \mathrm{~m}$ and the size of the slide is $24 \mathrm{~mm} = 0.024 \mathrm{~m}$ by $36 \mathrm{~mm} = 0.036 \mathrm{~m}$.
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