\left\{ {\left[ {\begin{array}{*{20}c}
 {x_1 } \\
 {x_2 } \\
 {x_3 } \\

 \end{array} } \right]:\left[ {\begin{array}{*{20}c}
 1 & 0 & { - 7} \\
 0 & 1 & { - 2} \\
 0 & 0 & 0 \\

 \end{array} } \right]\left[ {\begin{array}{*{20}c}
 {x_1 } \\
 {x_2 } \\
 {x_3 } \\

 \end{array} } \right] = \left[ {\begin{array}{*{20}c}
 0 \\
 0 \\
 0 \\

 \end{array} } \right]} \right\} = \left\{ {\left[ {\begin{array}{*{20}c}
 {7x_3 } \\
 {2x_3 } \\
 {x_3 } \\

 \end{array} } \right]:x_1 \in \mathbb{R}} \right\} = \left\langle {\left[ {\begin{array}{*{20}c}
 7 \\
 2 \\
 1 \\

 \end{array} } \right]} \right\rangle