A force parallel to the xx$-$axis acts on a particle moving along the xx$-$axis. This force is associated with potential energy U$($x$)$U$($x$)$ given by U$($x$)=$x$4$U$($x$)=$x$4,$ where $=0\mathrm{.}790$ J$/$m$4$J$/$m$4.$What is the magnitude of the force when the particle is at xx $=-0\mathrm{.}800$ mm$?$