I also said because they were trapeziums! As for this, if a is 4.5 then the area below the x-axis on the left hand side will be -2 units, the total area above is 3\frac {3}{4} then the area below on the right hand side is 1\frac {3}{4} so all together you get -2 + 15/4 - 7/4 = 0.