Horas.
If $y:=-x$, then $dy/dx=-1$ ... .
If $\displaystyle\frac{d}{dx}f(x)=g(x)$, then $\displaystyle\frac{d}{dy}f(y)=g(y)$, because $x$ and $y$ are both dummy variables ... .
$\displaystyle\frac{d}{dx}f(-x)=\frac{d}{dx}f(y)=\frac{dy}{dx}\frac{d}{dy}f(y)=-g(y)=-g(-x)$ ... .
Thus, this is correct ... .

Dalam Nama Bapa dan Putera dan Roh Kudus. Amin.