The vectors $$\vec{v}_1 = \left[\begin{array}{c} 2\cr -3\cr 0 \end{array}\right], \ \ \ \ \vec{v}_2 = \left[\begin{array}{c} -6\cr 6\cr 7 \end{array}\right], \ \ \ \ \vec{v}_3 = \left[\begin{array}{c} 0\cr -3\cr k\cr \end{array}\right]$$ form a basis for ${\mathbb R}^3$ if and only if $k\ne$ .