a) $\{\begin{array}{l}\frac{x-y}{2}+\frac{x+y}{3}=4\\ 2x-5y=4\left(x-3\right)+\left(1-2y\right)\cdot 3\end{array}$  

$\{\begin{array}{l}6\left(x-y\right)+4\left(x+y\right)=48\\ 2x-5y=4x-12+3-6y\end{array}$

$\{\begin{array}{l}y=5x-24\\ 2x-5y=4x-6y-9\end{array}$

$2x-25x+120=4x-9-30x+144$