Answer
$\frac{dy}{dx} = 3cos u=3cos(3x+1)$
Work Step by Step
$\frac{dy}{du} = \frac{d(sinu)}{dx} = cos u$
and
$\frac{du}{dx} = \frac{d(3x+1)}{dx} = 3$
So,
$\frac{dy}{dx} = \frac{dy}{du}*\frac{du}{dx}$
$\frac{dy}{dx} = 3cos u$
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