When the box is released , it will do SHM ,because by Hook's law restoring force is directly proportional to displacement and opposite in direction now velocity v in SHM for displacement x is given by v=\\omega\\sqrt{{a}^{2}-{x}^{2}} where \\omega = angular frequency and a= amplitudefrom above relation it is clear that velocity v will be maximum at a point where x is minimum i.e. x=0 and x=0 shows the mean position B .