[tex] \left\{\begin{array}{ccc}x+y=11\\-x=-y-9&|+y\end{array}\right\\\underline{+\left\{\begin{array}{ccc}x+y=11\\-x+y=-9\end{array}\right}\ \ \ |\text{add both sides of the equations}\\.\ \ \ \ \ 2y=2\ \ \ |:2\\.\ \ \ \ \ \ y=1\\\\\text{substitute the value of y to the first equation}\\\\x+1=11\ \ \ |-1\\x=10\\\\Answer:\ x=10;\ y=1\to(10;\ 1) [/tex]
