Your opponent only needs to assign 2 damage to Fog Bank because that's considered lethal damage even though its later prevented. The remeaning 5 will go on your life total because of trample
Actually, the controller of the creature with trample doesn't HAVE to assign the remaining damage on the defending player, he or she can still assign all the damage to the fog bank, it's just not the best thing to do, usually.
Rules question? Have you read the Basic rulebook already? No? Why not take some time to do that?