[tex]\displaystyle\bf\\\left \{ {{7x-y=0} \atop {3x-y=12}} \right. \\\\\\\left \{ {{y=7x} \atop {3x-7x=12}} \right. \\\\\\\left \{ {{y=7x} \atop {-4x=12}} \right. \\\\\\\left \{ {{x=-3} \atop {y=7\cdot(-3)}} \right.\\\\\\\left \{ {{x=-3} \atop {y=-21}} \right. \\\\\\Otvet \ : \ (-3 \ ; \ -21)[/tex]