$A x=\left[\begin{array}{rrr}a_{1,1} & a_{1,1} & a_{1,1} \\ a_{1,1} & a_{1,1} & a_{1,1} \\ a_{1,1} & a_{1,1} & a_{1,1}\end{array}\right]\left[\begin{array}{l}x_{1} \\ x_{2} \\ x_{3}\end{array}\right]=\left[\begin{array}{l}x_{1} \\ x_{1}+x_{2}+x_{3} \\ x_{3}-x_{2}\end{array}\right]$