{3x+2y=123x−y=3⇒{(3x+2y)−(3x−y)=12−33x=y+3

⇒⎧⎨⎩3y=9x=y3+1⇒{y=3x=2

=> 4x - 8y = -16 => 2a = 16 => a = 8

\(\left\{{}\begin{matrix}3x+2y=12\\3x-y=3\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}\left(3x+2y\right)-\left(3x-y\right)=12-3\\3x=y+3\end{matrix}\right.\)

\(\Rightarrow\left\{{}\begin{matrix}3y=9\\x=\dfrac{y}{3}+1\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}y=3\\x=2\end{matrix}\right.\)

=> 4x - 8y = -16 => 2a = 16 => a = 8