Chapter 9.1, Problem 11E

In Exercises 9-12, write the system of linear equations represented by the augmented matrix. Use x, y, and ~. or, if necessary, w, x, y. and z, for the variables.

$\left[\begin{array}{llll}1\hfill & 1\hfill & 4\hfill & 1\hfill \\ -1\hfill & 1\hfill & -1\hfill & 0\hfill \\ 2\hfill & 0\hfill & 0\hfill & 5\hfill \\ 0\hfill & 0\hfill & 12\hfill & 4\hfill \end{array}|\begin{array}{l}3\\ 7\\ 11\\ 5\end{array}\right]$