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Area of a trapezium =  1/2 (b + a) h
where  a and b are parallel sides and h is the altitude or distance between a and b.

In a canal, the floor of canal and the top water surface are parallel.  Height of trapezium is the depth of canal.

84 meters  =  1/2 ( 12 + 8) Height
Height = 168/20 = 8.4 meters

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