iTutor Chemistry Updates

Right so I've been starring at this thing for some hours - looking up some of the terms and doing a bit of research. I think I might have a decent idea how to do most of these. But I'm puzzled on something See this equation there in the middle - I'm thinking I could just half those values to answer question (b). But what's got me puzzled is how my lecturer got the 128 Kj mol-1? I've played with the numbers she's provided - every which way but I can't get the - bonds broken/formed here to add up to 128 Kj mol-1