The force acting on a particle moving along x–aixs varies with the position of the particle shown in the figure then body is in stable equilibrium at 

If particle at  $x_2$ is displaced to right, the force on it is negative and is brought back to $x_2$ . Similarly if particle is displaced to left, the force on it is positive and is again brought back to  $x_2$. Hence the particle is in stable equilibrium at $x_2$ .