[tex] \left\{\begin{array}{ccc}3x-7y=20&|\cdot10\\11x+10y=5&|\cdot7\end{array}\right\\\underline{+\left\{\begin{array}{ccc}30x-70y=200\\77x+70y=35\end{array}\right}\ \ \ \ |\text{add both sides of the equations}\\\\.\ \ \ \ \ 107x=235\ \ \ \ |:107\\.\ \ \ \ \ \ \ \ \ \ x=\dfrac{235}{107}\\\\\text{substitute the value of x to the second equation}\\\\11\cdot\dfrac{235}{107}+10y=5\\\\\dfrac{2585}{107}+10y=\dfrac{535}{107}\ \ \ \ |-\dfrac{2585}{107}\\\\10y=-\dfrac{2050}{107}\ \ \ \ |:10\\\\y=-\dfrac{205}{107} [/tex]
[tex] Answer:\ x=\dfrac{235}{107},\ y=-\dfrac{205}{107} [/tex]
