[tex] \left \{ {{y=2x} \atop {3x+2y=21}} \right \longrightarrow \left \{ {{y=2x} \atop {3x+2\cdot2x=21}} \right. \longrightarrow \left \{ {{y=2x} \atop {7x=21}} \right.\longrightarrow \left \{ {{y=2x} \atop \boxed{{x=3}}} \right. \\\\y=2x\longrightarrow y=2\cdot3\longrightarrow \boxed{y=6}[/tex]
