Solution
= Solution
{c}
Choose the <rotation> taking $e_1$ to $e_2$, $e_2$ to $-e_1$ and $e_3$ to itself. The columns of the <matrix of a linear map> are these images of the <standard basis>, so
$$
\boxed{A=\begin{pmatrix}0&-1&0\\1&0&0\\0&0&1\end{pmatrix}.}
$$
This fixes the orientation used throughout this question.