Viszlát is the shorter form of viszontlátásra, it is not so formal. But when you want to be polite, use the longer form. People will be happy to hear that ;) When you say goodbye to your friends (or to younger people), use Szia (or sziasztok - plural) instead.
I am trying to figure out why I was told to pay attention to the accents and I was marked correct with the accent on the a just like in the answer. Ok, now I see it. The I has an accent on it. They could make it a little bigger!