one question first: In your text you state that the inclination of the 80'-side is 45° but in your sketch the inclination is given with 40°. So which value is correct?
I'll use the 45° inclination. If this is wrong replace the 45 by 40.
1. Draw a sketch with all measures.
2. You've got 2 right triangles. The left one is an isosceles right triangle with d = x. Therefore
3. With the right one you'll get:
4. The volume of the complete ditch is a prism with the orange area as base area and the height of 200'. So you have first to determine the value of the orange area which is
The area of the trapezium is calculated by:
5. The volume of the complete ditch is now:
6. Now you have to transform the number of into because I'm not familiar with imperial measures. Sorry! (Maybe this is correct: )
7. Afterwards calculate the number of loads and consequently the costs for the transport.