\left \{ {{x+y=9} \atop {x-y=3}} \right.
\left \{ {{x=9-y} \atop {9-y-y=3}} \right.
\left \{ {{x=9-y} \atop {9-2y=3}} \right.
\left \{ {{x=9-y} \atop {-2y=3-9}} \right.
\left \{ {{x=9-y} \atop {-2y=-6}} \right.
\left \{ {{x=9-y} \atop {y=3}} \right.
\left \{ {{x=9-3} \atop {y=3}} \right.
\left \{ {{x=6} \atop {y=3}} \right.
…
\left \{ {{3x + 2y = 2 } \atop {- 3 x + y = 10}} \right.
\left \{ {{3x + 2y = 2 } \atop {y = 10+3x}} \right.
\left \{ {{3x + 2(10+3x) = 2 } \atop {y = 10+3x}} \right.
\left \{ {{3x + 20+6x = 2 } \atop {y = 10+3x}} \right.
\left \{ {{9x= 2-20 } \atop {y = 10+3x}} \right.
\left \{ {{9x=-18} \atop {y = 10+3x}} \right.
\left \{ {{x=-2} \atop {y = 10+3*(-2)}} \right.
\left \{ {{x=-2} \atop {y = 10-6}} \right.
\left \{ {{x=-2} \atop {y =4}} \right.