[tex]G(\frac{1}{3};1)\Rightarrow \overrightarrow{GA}=(-\frac{4}{3};2);\: \: \overrightarrow{GB}=(-\frac{4}{3};0);\: \: \overrightarrow{BC}=(4;-2)[/tex]
[tex]\overrightarrow{BC}=x.\overrightarrow{GA}+y.\overrightarrow{GB}\Rightarrow \left\{\begin{matrix} 4=-\frac{4}{3}x-\frac{4}{3}y & \\ -2= 2x+0.y& \end{matrix}\right.[/tex] [tex]\Rightarrow \left\{\begin{matrix} x=-1 & \\ y=-2 & \end{matrix}\right.[/tex]
[tex]\Rightarrow \overrightarrow{BC}=-\overrightarrow{GA}-2.\overrightarrow{GB}[/tex]