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

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

Vậy hệ có nghiệm duy nhất là ( 10; 7).

b) \(\left\{\begin{matrix} & 7x-3y =5\\ & 4x+ y = 2 \end{matrix}\right.\Leftrightarrow\) \(\left\{\begin{matrix} &7x- 3y = 5\\ & y = 2-4x \end{matrix}\right.\Leftrightarrow\) \(\left\{\begin{matrix} &7x- 3(2-4x) = 5\\ & y = 2-4x \end{matrix}\right.\Leftrightarrow\) \(\left\{\begin{matrix} &19x= 11\\ &y = 2- 4x\end{matrix}\right.\Leftrightarrow\)\(\left\{\begin{matrix} & x= \dfrac{11}{19};\\ & y = - \dfrac{6}{19}\end{matrix}\right. \)

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

c) \(\left\{\begin{matrix} & x+3y =-2\\ & 5x-4y = 11 \end{matrix}\right.\Leftrightarrow\) \(\left\{\begin{matrix} &x= -2- 4y\\ & 5( -2-3y) - 4y =11\end{matrix}\right.\Leftrightarrow\) \(\left\{\begin{matrix} & x = 02- 3y\\ & -10- 19y =11\end{matrix}\right.\Leftrightarrow\) \(\left\{\begin{matrix} & x= \dfrac{25}{19};\\ & y = - \dfrac{21}{19}\end{matrix}\right. \)

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