We have that the focal length is
[tex]f=19.058[/tex]
From the Question we are told that
image distance [tex]v=12.9[/tex]
object distance [tex]u=32.4[/tex]
Generally the equation for focal length is mathematically given as
[tex]\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\\\\\frac{1}{f}=\frac{1}{12}-\frac{1}{32.4}[/tex]
[tex]f=19.058[/tex]
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