I'm not sure I could do the math on this. Also though, I'm not sure I understand the question. When you say the "straight" line, do you mean the Normoglyc one? And you want to extrapolate out to hour 5? To get an exact number, I think you'd have to have the math of the curve. Visually I'd say it levels out at 105 mg per dl.
You title says "area," but I'm not sure that it makes sense to calculate area of this type of graph. It's not like a normal bell curve, were the x-axis represents members of a population. The graph is three different groups of people over time.
Again, not sure I understand.