Answer :
[tex] \left \{ {{y=6-2x} \atop {7x-y=3}} \right. \\\\substitute\ first\ equation\ into\ the\ second\\\\
7x-(6-2x)=3\\\\
7x-6+2x=3\\\\
9x-6=3\ \ \ | add\ 6\\\\
9x=9\ \ \ | divide\ by\ 9\\\\
x=1\\\\
y=6-2*1=6-2=4[/tex]