Imagine that there was no dip in the top and it was just a 31 by 69 rectangle. the area of the rectangle is 31 times 69 which is 2,139. The dip in the middle top part of the rectangle has an area as well which is 12 by 23 or 276 and if you subtract that area from the total area you get 2139-276= 1863 m^2