y=\left [\begin{array}{cccc} 2 \\ 8 \\ 6 \\ -4 \end{array} \right ] \quad \Longrightarrow P^{-1}y=\left [\begin{array}{cccc} 8 \\ -4 \\ 2 \\ 6 \end{array} \right ]
\Longrightarrow L^{-1}P^{-1}y=\left [\begin{array}{cccc} 8 \\ 0 \\ 2 \\ \frac{15}{2} \end{array} \right ]