1. (10%) Use Gauss-Jordan reduction to solve the following system:

$$\left\{ \begin{array}{l} x _ {1} - 2 x _ {2} - x _ {3} + 2 x _ {4} = 0 \\ 2 x _ {1} - x _ {2} + 3 x _ {3} + x _ {4} = 9 \\ - x _ {1} + x _ {2} - x _ {3} - x _ {4} = - 4 \end{array} \right.$$