a/ thay m=3 vào hệ ta có:
\(\left\{{}\begin{matrix}x+2y=12\\4x-3y=-7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}4x+8y=48\\4x-3y=-7\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}11y=55\\x+2y=12\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=5\\x=2\end{matrix}\right.\)
vậy..