Answer
The force is 1.29 N and it is directed toward the +x-axis.
Work Step by Step
$F = -\frac{dU}{dx}$
$F = -4\alpha ~x^3$
$F = (-4)(0.630~J/m^4)~x^3$
$F = (-2.52~J/m^4)~x^3$
We can find the force when the particle is at x = -0.800 m.
$F = (-2.52~J/m^4)~x^3$
$F = (-2.52~J/m^4)(-0.800~m)^3$
$F = 1.29~N$
The force is 1.29 N and it is directed toward the +x-axis.
