1. **State the problem:** We need to find the volume of a rectangular prism building with a smaller rectangular section cut out from the top corner. The main prism has dimensions 40 m by 40 m by 30 m, and the cut-out section is a cube with side length 9 m. 2. **Formula for volume of a rectangular prism:** $$\text{Volume} = \text{length} \times \text{width} \times \text{height}$$ 3. **Calculate the volume of the main prism:** $$40 \times 40 \times 30 = 48000 \text{ cubic meters}$$ 4. **Calculate the volume of the cut-out section:** Since the cut-out is a cube with side 9 m, $$9 \times 9 \times 9 = 729 \text{ cubic meters}$$ 5. **Calculate the volume of the building after the cut-out:** $$48000 - 729 = 47271 \text{ cubic meters}$$ 6. **Final answer:** The volume of the building is $$\boxed{47271} \text{ cubic meters}$$