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Top and bottom
Measure la and da. Measure lb and db. This will give you the area a and b for the top and bottom.
Measure the height (h) vertically and not alongside the edge.
The formula for a pyramidal frustum might seem difficult, and I guess coming up with the formula in the first place was. As it is, it's no harder to use than any other formula. Put the right numbers into the right slots, and you are there. The a is da multiplied with la to give the area. The b is db multiplied with lb to give the area.