Ta có:
\(\left\{{}\begin{matrix}-5x=7y\\3x+4y=-15\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=-\frac{7}{5}y\\3.\left(-\frac{7}{5}y\right)+4y=15\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=-\frac{7}{5}y\\-\frac{21}{5}y+4y=15\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=-\frac{7}{5}y\\-\frac{1}{5}y=15\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=-\frac{7}{5}y\\y=-75\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=105\\y=-75\end{matrix}\right.\)
Vậy x = 105 và y = -75.
\(3x+4y=-15\Leftrightarrow15x+20y=-75\Leftrightarrow-3.\left(-5x\right)+20y=-75\Leftrightarrow-21y+20y=-75\Leftrightarrow y=75\Leftrightarrow x=-105\)
