\(\left\{{}\begin{matrix}3\left(x-y\right)-5\left(x+2y\right)=99\\x-3y=7x-4y-17\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}3x-3y-5x-10y=99\\x-3y-7x+4y=-17\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}-2x-13y=99\\-6x+y=-17\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-6x-39y=297\\-6x+y=-17\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}-6x-39y+6x-y=297+17\\-6x+y=-17\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}-40y=314\\-6x=-17-y\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=-\dfrac{157}{20}\\x=\dfrac{y+17}{6}=\dfrac{\dfrac{157}{20}+17}{6}=\dfrac{497}{120}\end{matrix}\right.\)
\(\left\{{}\begin{matrix}3\left(x-y\right)-5\left(x+2y\right)=99\\x-3y=7x-4y-17\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}-2x-13y=99\\-6x+y=-17\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}-2x-13\left(6x-17\right)=99\\y=6x-17\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}-80x=-122\\y=6x-17\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{61}{40}\\y=-\dfrac{157}{20}\end{matrix}\right.\)
