Answer
$\frac{df}{dt}=3\frac{dx}{dt}$
Work Step by Step
$\frac{d}{dt}[f]=\frac{d}{dt}(3x)=\frac{d}{dx}(3x)\times\frac{dx}{dt}= 3\times\frac{dx}{dt}$
A related-rates equation is
$\frac{df}{dt}= 3\frac{dx}{dt}$
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