Consider this option:
[tex] \left \{ {{y=3x-4} \atop {2x+3y=21}} \right. \ =\ \textgreater \ \ \left \{ {{y=3x-4} \atop {2x+3(3x-4)=21}} \right. \ =\ \textgreater \ \ \left \{ {{y=3x-4} \atop {11x=33}} \right. \ =\ \textgreater \ \ \left \{ {{x=3} \atop {y=5}} \right. [/tex]
answer: (3;5).
