Решите систему уравнения способом сложения 6 класс(только 2 и 4)

$\left \{ {{8x-9y-21=0|*2} \atop {3x-2y-12=0|*(-9)}} \right. \\ + \left \{ {{16x-18y-42=0} \atop {-27x+18y+108=0}} \right. \\ \\ -11x+66=0 \\ 11x=66 \\ x=6 \\ \\ \left \{ {{x=6} \atop {3x-2y-12=0}} \right. \\ \left \{ {{x=6} \atop {18-12=2y}} \right. \\ \left \{ {{x=6} \atop {2y=6}} \right. \\ \left \{ {{x=6} \atop {y=3}} \right. \\ \\ OTBET:(6; 3)$
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$\left \{ {{x-8y-17=0} |*(-3)\atop {3x+4y-23=0}} \right. \\ +\left \{ {{-3x+24y+51=0} \atop {3x+4y-23=0}} \right. \\ \\ 28y+28=0 \\ 28y=-28 \\ y=-1 \\ \\ \left \{ {{y=-1} \atop {x-8y-17=0}} \right. \\ \left \{ {{y=-1} \atop {x=17+8*(-1)}} \right. \\ \left \{ {{y=-1} \atop {x=9}} \right. \\ \\ OTBET:(9; -1)$