This is why I've never liked estimation problems - there is not a single correct answer. Both your answer and their answer are good estimates, so both are correct. Your method works out to be better than theirs, at least in terms of accuracy. I suppose that the person who came up with this problem felt that using the point (400,40) as the corner of the trapezoid just looks better than point (360,40), maybe because the value at t=400 is close to 40 m/s (closer than what your line indicates), so maybe he felt that would give a better estimate. If not for the way the graph flattens out toward the end their answer would be better than yours.