Giải hệ phương trình sau: \(\left\{\begin{array}{l} 5(x+2 y)-3(x-y)=99 \\ x-3 y=7 x-4 y-17 \end{array}\right.\).

* Hướng dẫn giải

 \(\begin{array}{l} \text { Ta có }\left\{\begin{array}{l} 5(x+2 y)-3(x-y)=99 \\ x-3 y=7 x-4 y-17 \end{array} \Leftrightarrow\left\{\begin{array}{l} 5 x+10 y-3 x+3 y=99 \\ x-3 y-7 x+4 y=-17 \end{array} \Leftrightarrow\left\{\begin{array}{l} 2 x+13 y=99 \\ -6 x+y=-17 \end{array}\right.\right.\right. \\ \Leftrightarrow\left\{\begin{array}{l} 6 x+39 y=297 \\ -6 x+y=-17 \end{array} \Leftrightarrow\left\{\begin{array}{l} -6 x+y=-17 \\ 40 y=280 \end{array} \Leftrightarrow\left\{\begin{array}{l} y=7 \\ x=4 \end{array}\right.\right.\right. \end{array}\)