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