To make it real simple if the elevation is equal side to side and the slope is consistant end to end. Looking at a side profile you would have 2 equal triangles, one on the upper half of slope that would be flipped over on the lower half of slope. You multiply 3.5' x width x length of the upper half of area, then you divide it by 2. Look at a side profile of the measurement it is a rectangle, you need a triangle of that area and it is half of the rectangle area.