\documentclass[a4paper,twoside,12pt]{report}
\usepackage{graphicx}
\usepackage{a4wide}
\usepackage{wrapfig}

\DeclareGraphicsExtensions{.epsi,.eps,.ps,.eps.gz,.ps.gz,.gif,.epsi.gz}
\DeclareGraphicsRule{.ps.gz}{eps}{.ps.bb}{`gunzip -c #1}
\DeclareGraphicsRule{.eps.gz}{eps}{.eps.bb}{`gunzip -c #1}  

\begin{document}

\noindent \textbf{Rotaties oefenen}
\begin{description}
\item{a)}
\begin{eqnarray*}
\vec{\nabla} \times (2z , 3y, -x) &=&
\left( \begin{array}{c} \partial_x \\ \partial_y \\ \partial_z \end{array} \right)  \times
\left( \begin{array}{c} 2z \\ 3y \\ -x \end{array} \right)\\
&=& \left( \begin{array}{c} 0 \\ 3 \\ 0 \end{array} \right) \\
\end{eqnarray*}

\item{b)}
\begin{eqnarray*}
\vec{\nabla} \times (z^3 , 0, 10) &=&
\left( \begin{array}{c} \partial_x \\ \partial_y \\ \partial_z \end{array} \right) \times
\left( \begin{array}{c} z^3 \\ 0 \\ 10 \end{array} \right)\\
&=& \left( \begin{array}{c} 0 \\ 3z^2 \\ 0 \end{array} \right) \\
\end{eqnarray*}
\end{description}



\end{document}