Why are you double posting? You have already asked how to find the roots of the second derivative of this function, to which I offered a suggestion.
Double posting creates extra work for the moderators, which takes them away from giving help to those who need it, including yourself. It clutters the forum, and also creates the potential for the duplication of effort of the part of those who help. Please post your questions once, then wait patiently for help, and take the suggestions given, or at least ask for clarification if the given suggestion doesn't give you a direction in which to go.