Answer
See solution
Work Step by Step
Any vector x such that $x=\begin{bmatrix}
1\\
-2\\
1\\
0
\end{bmatrix}x_3+\begin{bmatrix}
-1\\
4\\
0\\
1
\end{bmatrix}x_4
$ where $x_3$ and $x_4$ are free variables will be in Nul A.
One example is $\begin{bmatrix}
0\\
2\\
1\\
1
\end{bmatrix}
$
Any x such that $x=\begin{bmatrix}
3\\
-9\\
9\\
\end{bmatrix}c_1+\begin{bmatrix}
2\\
-4\\
2\\
\end{bmatrix}c_2+\begin{bmatrix}
1\\
1\\
-5\\
\end{bmatrix}c_3+\begin{bmatrix}
-5\\
7\\
1\\
\end{bmatrix}c_4
$ where $c_1,c_2,c_3,c_4$ are scalars is in Col A. One example is $\begin{bmatrix}
1\\
1\\
-5\\
\end{bmatrix}$