Write the system of linear equations represented by the augmented matrix. (Use the variables $x, y, z,$ and $w,$ if applicable.)
$$\left[\begin{array}{rrrrrr}
6 & 2 & -1 & -5 & 5 & -25 \\
-1 & 0 & 7 & 3 & \vdots & 7 \\
4 & -1 & -10 & 6 & \vdots & 23 \\
0 & 8 & 1 & -11 & \vdots & -21
\end{array}\right]$$