#### Answer

$\frac{dy}{dx} = e^{-x}sin u=e^{-x}sin(e^{-x})$

#### Work Step by Step

$\frac{dy}{du} = \frac{d(cos u)}{du} = -sin u$
and,
$\frac{du}{dx} = \frac{d(e^{-x})}{dx} = -e^{-x}$
So,
$\frac{dy}{dx} = \frac{dy}{du}*\frac{du}{dx} $
$\frac{dy}{dx} = e^{-x}sin u$