I just thought I understood it, but it is really complicated. I have read from many sites, but couldn't find a common patern to explain it. On many sites it is like bossza wrote, EA1 negative, EA2 positive, but on the USNCO test it is written opposite, and as a note it says for the first EA: "A positive EA value is exothermic", but in calculations the positive value is still used. It shouldn't be a mistake when so many people took part in making those problems, but I couldn't a good explanation. Really complicated.