A certain spring is found not to obey Hooke's law; it exerts a restoring force Fx$($x$)=$x$$x$2$Fx$($x$)=$x$$x$2$ if it is stretched or compressed, where $=60\mathrm{.}0$N$/$m$=60\mathrm{.}0$N$/$m and $=18\mathrm{.}0$N$/$m$2$$=18\mathrm{.}0$N$/$m$2.$ The mass of the spring is negligible