\(\left\{{}\begin{matrix}-a+b=6\\2a+b=-3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}3a=-9\\-a+b=6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=-3\\b=3\end{matrix}\right.\)
\(\left\{{}\begin{matrix}-a+b=6\\2a+b=-3\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}-a+b=6\\-a+b-2a-b=6-\left(-3\right)\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}-a+b=6\\-3a=9\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}-\left(-3\right)+b=6\\a=-3\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}b=3\\a=-3\end{matrix}\right.\)
