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$\left \{ {{y=3x-8} \atop { x^{2} +4x(3x-8)=21}} \right.$

$\left \{ {{y=3x-8} \atop { x^{2} +12 x^{2} -32x=21}} \right.$

$\left \{ {{y=3x-8} \atop {13 x^{2} -32x-21=0}} \right.$

$\left \{ {{x= 3} \atop {y=3*3-8}} \right.$ или $\left \{ {{x=- \frac{7}{13} } \atop {y=3*(- \frac{7}{13})-8 }} \right.$

$\left \{ {{x=3} \atop {y=1}} \right.$ или $\left \{ {{x=- \frac{7}{13} } \atop {y = -9 \frac{8}{13} }} \right.$