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The semicircle has diameter 18 ft, so it has radius 9 ft, and this is the height of the rectangle.

The area of the shaded region is (area of rectangle) - (area of semicircle).

The area of a circle with radius is , so the area of a semicircle with the same radius is , and in this case that's .

The area of the rectangle is .

Then the area of the shaded region is