Conservative and Non-Conservative Forces (Problem 29)

A particle of mass $2.0\text{kg}$ moves under the influence of the force $F(x)=(-5x^2+7x)\text{N}.$ If its speed at $x=-4.0\text{m}$ is $v=20.0\text{m/s,}$ what is its speed at $x=4.0\text{m}?$