------
## 最终结论
```python
def surfacearea_cuboid(length, width, height):
    return 2 * (length * width + length * height + width * height)

# Test cases
assert surfacearea_cuboid(1, 2, 3) == 22
assert surfacearea_cuboid(5, 7, 9) == 286
assert surfacearea_cuboid(10, 15, 21) == 1350
```