I have a further question regarding the calculation of area for landscape slopes.

Included is the further example where one end of my prism has differing dimensions to it's opposing end.

Take the example attached:

C1 = 3.9
C2 = 1.05
Top perimeter = 20.5
Bottom Perimeter = 19

How do I calculate the area of x?

Using the same principles as before:-

{ .5 ( 3.9*1.5) } * { .5 (20.5*19)}

Calculate this and you will get your answer.