To find the area of the whole shape, you gotta divide it into polygons. So, you find the area of the big rectangle at the bottom first: 18 x 7 = 126. That's the area of the big rectangle.
Then, you find the area of the little square on top. 4 x 4 - 16, that's the area. Then, you gotta add those two together: 126 + 16. The total area is 142 square feet.