$\left\{\begin{array} { l } x+y=58 \\ x-y=12\end{array} \right.$
$\left\{\begin{array} { l } x=58-y \\ x-y=12\end{array} \right.$
$\left\{\begin{array} { l } x=58-y \\ x=12+y\end{array} \right.$
$58-y=12+y$
$y=23$
$x=12+23$
$x=35$
$\left( x, y\right)=\left( 35, 23\right)$
$\left\{\begin{array} { l } 35+23=58 \\ 35-23=12\end{array} \right.$
$\left\{\begin{array} { l } 58=58 \\ 12=12\end{array} \right.$
$\left( x, y\right)=\left( 35, 23\right)$