$\dfrac{x}{y}=-\dfrac{2}{3}$
$↔\dfrac{x}{-2}=\dfrac{y}{3}$
Đặt $\dfrac{x}{-2}=\dfrac{y}{3}=k$
$→\begin{cases}x=-2k\\y=3k\end{cases}$
Thay $x=-2k$ và $y=3k$ vào $A$:
$A=\dfrac{21.(-2k)-14.3k}{73.(-2k)+73.3k}$
$=\dfrac{-42k-42k}{-146k+219k}$
$=\dfrac{-84k}{73k}=-\dfrac{84}{73}$