Answer
$\frac{dy}{dx} = (1 - \frac{x}{7})^{-8}$
Work Step by Step
$y = u^{-7}$
where,
$u = (1 - \frac{x}{7})$
Now,
$\frac{dy}{du} = -7u^{-8}$
and,
$\frac{du}{dx} = -\frac{1}{7}$
So,
$\frac{dy}{dx} = \frac{dy}{du} \times \frac{du}{dx}$
$\frac{dy}{dx} =u^{-8}$
$\frac{dy}{dx} = (1 - \frac{x}{7})^{-8}$
