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\(\left\{\begin{matrix}-9x+9y=-126\\3y+6x=21\end{matrix}\right.\)

\(\left\{\begin{matrix}-9x+9y=-126\\3y+6x=21\end{matrix}\right.\\ \Leftrightarrow \left\{\begin{matrix}-9x+9y=-126\\6x+3y=21\end{matrix}\right.\text{(Niet verplichte stap, proper schrijven)}\\ \Leftrightarrow \left\{\begin{array}{c|c|c}-9x+9y=-126& \color{red}{2.} & \color{blue}{1.} \\6x+3y=21& \color{red}{3.} & \color{blue}{-3.} \end{array}\right.\\ \Leftrightarrow \left\{\begin{matrix} \color{red}{\underline{-18x+18x}+18y+9y=-252+63} \\ \color{blue}{-9x-18x\underline{+9y-9y}=-126-63} \end{matrix}\right.\\ \Leftrightarrow \left\{\begin{matrix}27y=-189 \\-27x=-189\end{matrix}\right.\\ \Leftrightarrow \left\{\begin{matrix}y=\frac{-189}{27}=-7 \\x=\frac{-189}{-27}=7\end{matrix}\right.\\ \qquad V=\{(7,-7)\}\)