Only the person who down voted you can say for sure, but if you mean this one:
How to calculate no of sundays between two dates[
^]
Then I'd guess it's because your answer was to a question that was posted in 2011 and in practice added nothing useful.
Answering old questions is a bad idea: it may seem like an easy way to get reputation points, but it is considered a form of abuse and may lead to you being banned from the site. I would strongly suggest that you keep to modern questions if you want to remain...