A particular spring obeys the force law \vec{F} = (-kx + ax^3 + bx^5)\hat{i}, where k, a, and b are constants. The spring is stretched out to a position $x_0$ from the equilibrium position. A mass m is attached to the spring and then released from rest.
Part A
What is the speed of m at $x = x_0$?
Express your answer in terms of some, all, or none of the constants k, a, and b, and variables m and $x_0$.