[tex]\left\{\begin{array}{ccc}x+y=11\\-x=-y-9&|\text{change the signs}\end{array}\right\\\left\{\begin{array}{ccc}x+y=11\\x=y+9\end{array}\right\\\\\text{substitute x=y+9 to the first equation}\\\\(y+9)+y=11\\y+9+y=11\ \ \ \ |\text{use commutative property}\\(y+y)+9=11\\2y+9=11\ \ \ \ |\text{subtract 9 from both sides}\\2y=2\ \ \ \ |\text{divide both sides by 2}\\\boxed{y=1}\\\\\text{put the value of y to the second equation}\\\\x=1+9\\\boxed{x=10}\\\\Answer:\ A.\ (10,\ 1)[/tex]
