$\left \{ {{x+3y=4} \atop {2x-3y=-1}} \right.$
$⇒$ $\left \{ {{2x+6y=8} \atop {2x-3y=-1}} \right.$
$⇔$ $\left \{ {{9y=9} \atop {2x-3y=-1}} \right.$
$⇔$ $\left \{ {{y=1} \atop {2x-3y=-1}} \right.$
$⇔$ $\left \{ {{y=1} \atop {x=1}} \right.$
Vậy `(x;y)=(1;1)`