For the following matrices, write the kernel as the span of a finite number of vectors. Is the kernel a point, line, plane, or all of $\mathbb{R}^3$ ?
(a) $\left(\begin{array}{ll}2 & -1\end{array}\right.$ 5),
(b) $\left(\begin{array}{rrr}1 & 2 & -1 \\ 3 & -2 & 0\end{array}\right)$,
(c)
$\left(\begin{array}{rrr}2 & 6 & -4 \\ -1 & -3 & 2\end{array}\right)$
(d)
$\left(\begin{array}{rrr}1 & 2 & 5 \\ 0 & 4 & 8 \\ 1 & -6 & -11\end{array}\right)$
(e) $\left(\begin{array}{rrr}2 & -1 & 1 \\ -1 & 1 & -2 \\ 3 & -1 & 1\end{array}\right)$,
(f)
$\left(\begin{array}{rrr}1 & -2 & 3 \\ -3 & 6 & -9 \\ -2 & 4 & -6 \\ 3 & 0 & -1\end{array}\right)$