Potential Energy Diagrams and Stability (Problem 47)

A particle of mass 4.0 kg is constrained to move along the x-axis under a single force $F(x)=\text{−}c{x}^3,$ where $c=8.0{\text{N/m}}^3.$ The particle’s speed at A, where $x_A=1.0\text{m,}$ is 6.0 m/s. What is its speed at B, where $x_B=-2.0\text{m?}$