\left \{ {{\frac{a}{b}=3} \atop {a+b=a-b+6}} \right.
\left \{ {{a=3b} \atop {3b+b=3b-b+6}} \right.
\left \{ {{a=3b} \atop {4b-2b=6}} \right.
\left \{ {{a=3b} \atop {2b=6}} \right.
\left \{ {{a=3b} \atop {b=3}} \right.
\left \{ {{a=3*3} \atop {b=3}} \right.
\left \{ {{a=9} \atop {b=3}} \right.