\mathbf{v} \times \mathbf{a} = \begin{pmatrix} y(4) - z(-1) \\ z(2) - x(4) \\ x(-1) - y(2) \end{pmatrix} = \begin{pmatrix} 4y + z \\ 2z - 4x \\ -x - 2y \end{pmatrix} = \begin{pmatrix} 5 \\ -6 \\ -1 \end{pmatrix}