[tex]\displaystyle\bf\\\left \{ {{5(x-y)=10} \ |:5\atop {3x-7y=20-(x+y)}} \right. \\\\\\\left \{ {{x-y=2} \atop {3x-7y=20-x-y}} \right. \\\\\\\left \{ {{x-y=2} \atop {3x+x-7y+y=20}} \right.\\\\\\\left \{ {{x-y=2} \atop {4x-6y=20} \ |:(-4)} \right.\\\\\\+\left \{ {{x-y=2} \atop {-x+1,5y=-5}} \right. \\-----------\\0,5y=-3\\\\y=-3:0,5=-6\\\\x=2+y=2+(-6)=2-6=-4\\\\\\Otvet: \ \Big(-4 \ ; \ -6\Big)[/tex]