I am trying to get this array
\begin{equation}U = \left( \begin{array}{c} \rho\\ \rho u\\ \rho v\end{array} \right),\end{equation}
into the values marked x in this array
\[ F^+(U) = \left\{ \begin{array}{ll} x & \mbox{$x \geq 0$};\\ x & \mbox{$x \geq 0$};\\ -x & \mbox{$x < 0$}.\end{array} \right. \]
so something like
\[ F^+(U) = \left\{ \begin{array}{ll} \begin{equation}U = \left( \begin{array}{c} \rho\\ \rho u\\ \rho v\end{array} \right),\end{equation} & \mbox{$x \geq 0$};\\ x & \mbox{$x \geq 0$};\\ -x & \mbox{$x < 0$}.\end{array} \right. \]
however this won't compile. Any help