A particle which is constrained to move along x-axis, is subjected to a force in the same direction which varies with the distance x of the particle from the origin as $\mathrm{F}\left(\mathrm{x}\right)=-\mathrm{kx}+{\mathrm{ax}}^3$. Here k and a are positive constant .for $\mathrm{x}\ge 0,$ the functional form of potential energy U (x) of the particle is