U=\left [\begin{array}{cccc} 2 & 2 & 4 & 2 \\ 0 & 2 & 1 & 6 \\ 0 & 0 & 2 & 2 \\ 0 & 0 & 0 & -\frac{7}{2} \end{array} \right ], \quad x=\left [\begin{array}{c} -1 \\ 2 \\ 2 \\ -1 \end{array} \right ].