Can I know who down voted my question/answer on Ask Ubuntu so that I can ask him the reason for it?
Voting on StackExchange sites is anonymous - even the community moderators do not have access to that information.
Try adding a comment to your question asking for feedback on how to improve the question.
Look for similar style questions - look at their content and see if you can add similar information to improve the question.
At the end of the day - sometimes you just get random down-votes for no apparent reason. It does balance out eventually with equally random upvotes, so try not to worry too much!