Use the given LU factorization to find all solutions to $\textrm{A}\textbf{x} = \textbf{b}$
$\textrm{A} = \left[\begin{array}{cccc} 1 &0 &0 &0\cr -6 &1 &0 &0\cr 1 &-6 &1 &0\cr 5 &9 &1 &1 \end{array}\right] \left[\begin{array}{ccc} -5 &4 &-2\cr 0 &2 &-2\cr 0 &0 &4\cr 0 &0 &0 \end{array}\right]$, $\textbf{b}=\left[\begin{array}{c} -9\cr 68\cr -117\cr 57 \end{array}\right]$
$\textbf{x}=$$\left[\Rule{0pt}{3.6em}{0pt}\right.$$\left]\Rule{0pt}{3.6em}{0pt}\right.$