The coefficient for a3 = a1/3 + a0/6 because I had a3 = a1/3 + a2/3. So when I substituted for a2 I got what I just told you on the first line. Can you double check that one.
first of all, that is incorrect. second of all, your questions are very vague. you name variables out of nowhere that people who don't have your text book won't know what you are talking about, be more specific and explanatory.