i didnt get how they got the answer? how did they get 7

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I'll advise you to forget about the convolution stated in the lecture and think about it in one of these ways. Either you perceive it as two triangle and rectangular glued together and find their areas then add them up. Or if you hate geometry. just Sum the two parallel segments and divide them by 2 then multiply everything with the alititude. Or mathematically ((a+b)/2)*h wher a and b are parallel segments. Hope that might help you. I didn't want to dwell on it but hoping you get it.