Consider the subspace $$U=\text{span} \lbrace \left[\begin{array}{c} -2\cr -1\cr -4\cr 2 \end{array}\right], \left[\begin{array}{c} -1\cr 4\cr 4\cr 1 \end{array}\right], \left[\begin{array}{c} -5\cr 1\cr -7\cr 5 \end{array}\right] \rbrace$$ of $\mathbb{R}^4$. Create a basis $$\lbrace \left[\begin{array}{c} -1\cr 2\cr -2\cr 1 \end{array}\right],\left[\begin{array}{c} 1\cr 3\cr 2\cr -1 \end{array}\right],x \rbrace$$ for $U$.
$x =$ $\left[\Rule{0pt}{4.8em}{0pt}\right.$$\left]\Rule{0pt}{4.8em}{0pt}\right.$