Найдите значение выражения x-2y,если(x;y)-решение системы 4x=y=8,xy=4

Решите задачу:

$\left \{ {{4x+y=8,} \atop {xy=4}} \right. \ \left \{ {{y=8-4x,} \atop {x(8-4x)=4}} \right. \ \left \{ {{y=8-4x,} \atop {8x-4x^2-4=0;}} \right. \ \left \{ {{y=8-4x,} \atop {x^2-2x+1=0;}} \right. \ \left \{ {{y=8-4x,} \atop {(x-1)^2=0;}} \right. \\ \left \{ {{y=8-4x,} \atop {x-1=0;}} \right. \ \left \{ {{y=8-4x,} \atop {x=1;}} \right. \ \left \{ {{x=1,} \atop {y=4;}} \right. \\ x-2y=1-2\cdot4=1-8=-7.$