\left \{ {{2x-3y=3} \atop {x+y=4}} \right.
\left \{ {{2x-3y=3} \atop {x=4-y}} \right.
\left \{ {{2(4-y)-3y=3} \atop {x=4-y}} \right.
\left \{ {{8-2y-3y=3} \atop {x=4-y}} \right.
\left \{ {{-5y=3-8} \atop {x=4-y}} \right.
\left \{ {{-5y=-5} \atop {x=4-y}} \right.
\left \{ {{y=1} \atop {x=4-1}} \right.
\left \{ {{y=1} \atop {x=3}} \right.
…
\left \{ {{x+6y+2=13} \atop {x=y-3}} \right.
\left \{ {{y-3+6y+2=13} \atop {x=y-3}} \right.
\left \{ {{7y-1=13} \atop {x=y-3}} \right.
\left \{ {{7y=13+1} \atop {x=y-3}} \right.
\left \{ {{7y=14} \atop {x=y-3}} \right.
\left \{ {{y=2} \atop {x=2-3}} \right.
\left \{ {{y=2} \atop {x=-1}} \right.
…
\left \{ {{15x-\frac{1}{2}*y=-4} \atop {2x+y=8}} \right.
\left \{ {{15x-\frac{1}{2}*y=-4} \atop {y=8-2x}} \right.
\left \{ {{15x-\frac{1}{2}*(8-2x)=-4} \atop {y=8-2x}} \right.
\left \{ {{15x-\frac{1}{2}*2(4-x)=-4} \atop {y=8-2x}} \right.
\left \{ {{15x-(4-x)=-4} \atop {y=8-2x}} \right.
\left \{ {{15x-4+x=-4} \atop {y=8-2x}} \right.
\left \{ {{16x=-4+4} \atop {y=8-2x}} \right.
\left \{ {{16x=0} \atop {y=8-2x}} \right.
\left \{ {{x=0} \atop {y=8-2*0}} \right.
\left \{ {{x=0} \atop {y=8}} \right.