The question has to have been recent enough, so check the date, I believe the time span is within a weeks or so. But after that the option to answer is gone pretty much. But there would be a blue button with the word answer on it.
Under the person question there is a icon that says "answer" you click it then put down your answer. After that look to your top right and there is a "add your answer" icon click it and your answer is shared with other people.
