$\left. \begin{matrix}   2x+y+{{10}^{{}^\circ }}+{{60}^{{}^\circ }}={{180}^{{}^\circ }}  \\   x-2y+{{30}^{{}^\circ }}+{{120}^{{}^\circ }}={{180}^{{}^\circ }}  \\\end{matrix} \right\}\Rightarrow \left. \begin{matrix}   2x+y={{110}^{{}^\circ }}  \\   x-2y={{30}^{{}^\circ }}  \\\end{matrix} \right\}\Rightarrow 3x-y={{140}^{{}^\circ }}$