x/4+x/8=3
1)
{xy=-6
{x+2y=-1
{x=-1-2y
{y(1+2y)=6
{y+2y^2=6{2y^2+y-6=0
D=1-4*2*-6=1+48=7^2
y=-1+7/4=3/2
y2=-1-7/4=-2
x=-4
x2=3
2)
{5x-7y=31
{5x+7y=-11
{5x=31+7y
{-11y-7y=31+7y
{ -42=14y
{y=-3
{x=2
3)
{3x-11y=36
{3x+11y= -30
{3x=36+11y
{-30 -11y=36+11y
{-66 =22y
{x=1