MEI C2: Trapezium rule help

can anyone help me with the trapezium rule please?

I can use and apply the rule well but there are certain questions where I ALWAYS lose marks which is when they ask you to 'use rectangles to calculate a lower/higher bound'? What do I do to do this? e.g. January 2010 Q11ii)

http://www.mei.org.uk/files/papers/c210ja_4752_2.pdf

Also I'm really poor at explaining whether it's an over/underestimation - I know that a concave shape gives an overestimate and vice versa but can't explain this using a diagram or with any more detail than 'it's concave therefore an overestimate'. Could anyone help with this please?

You make the rectangle level with the left as you approach the peak and then after the peak draw rectangles level with the right of the points to make the rectangles at the minimum then can be. Then find the area like a normal rectangle.