I am having some difficulty calculating the volume of an earthworks batter (the sloping shoulder section of an embankment).

I have used the formula for calculating the volume of a pyramid and have also used a method of calculating the average area (in this case the area of one end divided by 2 as the area at the other is 0) and multiplying that by the length.

However the two results I get are vastly different! I have no idea why...

I believe the solution will be straight forward however I cannot figure it out solo and need the wisdom of someone more capable than myself to point me in the right direction.

Please see attached.

(clearly my word processing skills exceed my mathematical ones...)