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Fausto, That makes the calculations particularly nice. If the angle CAB were a right angle then you can use Pythagoras theorem on the triangle ABC and conclude that BC has length 13 feet. This gives the slope angle as 22.37 degrees rather than 24 degrees but I think that will give a sufficiently accurate approximation of the volume of fill you need. The volume of fill is the area of the triangle ACB times the length of the wall which is 90 feet. The area of the triangle is half the base times the height and since I am assuming the angle CAB is a right angle I can take the base to be AC and the height is then the length of AB. Thee measurements are all in feet and you want the volume in cubic yards so I am going to convert the lengths to yards by dividing each by 3.
I hope this helps,  


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