A small object with mass $m=0\mathrm{.}0900kg$ moves along the $+x-$axis. The only force on the object is a conservative force that has the potential$-$energy function $U(x)=-a{x}^2+b{x}^3,$ where $a=2\mathrm{.}00\frac{J}{m^2}$ and $b=0\mathrm{.}300\frac{J}{m^3}.$ The object is released from rest at small $x.$ When the object is at $x=4\mathrm{.}00m,$ what is its velocity?