It seems that there is a bit of misunderstanding here. This is what I gave gathered so far from your question, the correct answer to the first part is 478 kJ. You are not getting this answer correct and you want to know why, am I correct? You keep mentioning that '393 kJ and 242 kJ will be negative', do you have some sort of answer sheet which is telling you this or where are you getting this from? The way I see it, the problem can be solved by multiplying reaction one by 2, reaction two by 4 and flipping reaction three and multiplying by 2. This would give 2 x 393 + 242 x 4 - 638 x 2 which gives 478 kJ