a) \(\left\{\begin{matrix}x-y =3\\ 3x-4y= 2\end{matrix}\right. \Leftrightarrow \left\{\begin{matrix}y= x-3\\ 3x-4y = 2\end{matrix}\right. \)\( \Leftrightarrow \left\{\begin{matrix}y =x- 3\\ 3x- 4(x-3)= 2\end{matrix}\right. \Leftrightarrow \left\{\begin{matrix}y = x-3\\ -x+12= 2 \end{matrix}\right.\) \( \Leftrightarrow \left\{\begin{matrix}y=x-3\\ x=10\end{matrix}\right. \Leftrightarrow \left\{\begin{matrix}y=7\\ x=10\end{matrix}\right. \)

Vậy hệ có nghiệm duy nhất là( 10; 7)

b) \( \left\{\begin{matrix}7x- 3y = 5 \\ 4x+ y =2\end{matrix}\right. \Leftrightarrow \left\{\begin{matrix}7x-3y =5 \\ y = 2-4x\end{matrix}\right. \Leftrightarrow \left\{\begin{matrix}7x- 3(2-4x) =5 \\ y = 2- 4x\end{matrix}\right. \Leftrightarrow \left\{\begin{matrix} 19x= 11\\ y=2 - 4x\end{matrix}\right. \Leftrightarrow \left\{\begin{matrix} 19x= 11\\ y=2 - 4x\end{matrix}\right. \)

\( \Leftrightarrow \left\{\begin{matrix}x= \frac{11}{19}\\ y =- \frac{6}{19}\end{matrix}\right.\)

Vậy hệ có nghiệm duy nhất ( \(\frac{11}{19};- \frac{6}{19}\))

c) \( \left\{\begin{matrix} x+ 3y = -2\\ 5x-4 y =11 \end{matrix}\right. \Leftrightarrow \left\{\begin{matrix}x= -2 - 3y \\ 5(-2-3y) -4y = 11\end{matrix}\right. \Leftrightarrow \left\{\begin{matrix}x= -2 - 3y \\ -10-19y = 11\end{matrix}\right. \Leftrightarrow \left\{\begin{matrix}x= -\frac{21}{19} \\ y = \frac{25}{19}\end{matrix}\right.\)

Vậy hệ có nghiệm duy nhất ( \(\frac{25}{19};- \frac{21}{19}\))