Answer
$ f(x)$ is odd
Work Step by Step
$ f(x)$ is odd if $f(-x)=-f(x)$
$ f(x)$ is even if $f(-x)=f(x)$
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$f(x)=3x$
$f(-x)=3(-x)=-(3x)=-f(x)$
$ f(x)$ is odd
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