Start by calculating the volume. The bigger pyramid minus the smaller one (room/empty space)
The regular pyramid roof of the Washington Monument is 55 ft. high and has a base which is a square 35 ft. 2 in. on a side. The marble slabs of which it is built weigh 165 lb. per cu. ft. If the room covered by the roof is a pyramid whose square base is 34 ft. on a side and 54 ft. high, what is the weight of the roof? (Neglect the six small windows.)
you have to find the volume of the roof, so you take the outside limits, which are defined by the hight of 55' and a square base with a side of 35' 2" and the inside limits (because the room itself is empty and does not add to the weight, so you subtract its volume from the total)