I.
\left \{ {{x-y=2} \atop {2x+y=4}} \right.
\left \{ {{x=2+y} \atop {2(2+y)+y=4}} \right.
\left \{ {{x=2+y} \atop {4+2y+y=4}} \right.
\left \{ {{x=2+y} \atop {3y=4-4}} \right.
\left \{ {{x=2+y} \atop {3y=0}} \right.
\left \{ {{x=2+y} \atop {y=0}} \right.
\left \{ {{x=2+0} \atop {y=0}} \right.
\left \{ {{x=2} \atop {y=0}} \right.
…
\left \{ {{2x+y=0} \atop {-x+y=-3}} \right.
\left \{ {{y=-2x} \atop {-x-2x=-3}} \right.
\left \{ {{y=-2x} \atop {x+2x=3}} \right.
\left \{ {{y=-2x} \atop {3x=3}} \right.
\left \{ {{y=-2x} \atop {x=1}} \right.
\left \{ {{y=-2} \atop {x=1}} \right.
\left \{ {{x=1} \atop {y=-2}} \right.
…
II.
\left \{ {{2x+y=6} \atop {x=1}} \right.
\left \{ {{2*1+y=6} \atop {x=1}} \right.
\left \{ {{y=6-2} \atop {x=1}} \right.
\left \{ {{y=4} \atop {x=1}} \right.
…
\left \{ {{3x-y=11} \atop {y=-1}} \right.
\left \{ {{3x-(-1)=11} \atop {y=-1}} \right.
\left \{ {{3x+1=11} \atop {y=-1}} \right.
\left \{ {{3x=11-1} \atop {y=-1}} \right.
\left \{ {{3x=10} \atop {y=-1}} \right.
\left \{ {{x=\frac{10}{3}} \atop {y=-1}} \right.
\left \{ {{x=3\frac{1}{3}} \atop {y=-1}} \right.