an antiderivitive ask for a single +c value so you could assume that it 0 and hence not include it.
However marking schemes may change year by year so the most safe option is to write + c (shows the assessor you are aware of the constant of integration) and then say "therefore an antiderivitive is" _________ (answering the question directly).
Past years have indicated you don't need to say +c for "an antiderivitive" so just don't include it and assume 0
So the 'c' isnt necessary?
This is what I got
-c looks weird so i'd probably just change c to c1 -> c2 when manipulating
Also you changed c to -c but didn't change it when dividing by 3 it would be best to keep it consistent.
Not sure whether you would actually lose marks but you will definitely not lose marks if you just keep changing the c-c1,c2 .