Answer
$dy=-x(9-x^{2})^{-\frac{1}{2}} dx$
Work Step by Step
$y=\sqrt (9-x^{2})=(9-x^{2})^{\frac{1}{2}}$
$\frac{dy}{dx}=\frac{1}{2}(9-x^{2})^\frac{-1}{2}\times-2x$ (product rule, chain rule)
$=-x(9-x^{2})^\frac{-1}{2}$
$dy=-x(9-x^{2})^\frac{-1}{2}\times dx$
