The function you want to hook into is qa_answer_create() in qa-app-post-create.php. This is called whenever an answer is created. The $content parameter will contain the answer text, and you can also access the question title from the $question['title'] variable.
How will you know if it's the first answer to the question? The easiest way is to check that ($question['acount']==0) i.e. that the question had no answers before this new answer was added.
My recommendation would be to write this functionality in a separate PHP file, and add a single line into qa-app-post-create.php - that way it will be minimal work to apply it again to future versions of Question2Answer.