Answer
a basis for the nullspace of $A$ consists of the vectors
$$ \left[\begin{aligned} -3\\ 0\\1 \end{aligned}\right] .$$
Work Step by Step
Given the matrix
$$
A=\left[\begin{array}{lll}{1} & {2} & {3} \\ {0} & {1} & {0}\end{array}\right].
$$
The reduced row echelon form is given by
$$
\left[ \begin {array}{ccc} 1&0&3\\ 0&1&0
\end {array} \right]
.
$$
The corresponding system is
$$
\begin{aligned} x_{1} +3x_3&=0 \\
x_2&=0.\\
\end{aligned}.
$$
The solution of the above system is $x_1=-3t$, $x_2=0$ and $x_3=t$. This means that the solution space of $Ax = 0 $ consists of the vectors on the following form
$$x= \left[\begin{aligned} x_{1}\\ x_{2} \\x_3 \end{aligned}\right]= \left[\begin{aligned} -3t\\ 0\\t \end{aligned}\right]=t\left[\begin{aligned} -3\\ 0\\1 \end{aligned}\right] .$$
So, a basis for the nullspace of $A$ consists of the vectors
$$ \left[\begin{aligned} -3\\ 0\\1 \end{aligned}\right] .$$