Answer
$$[x]_S=\left[ \begin {array}{ccc} 1\\ -3\\ 0\end {array} \right].$$
Work Step by Step
Since we have
$$x= (1,0,0)-3(0,1,0)+0(0,0,1),$$
then the coordinate matrix of $x$ in $R^3$ relative to the
standard basis is
$$[x]_S=\left[ \begin {array}{ccc} 1\\ -3\\ 0\end {array} \right].$$