1. **Problem statement:** We are given a cuboid with a volume of 2457 mm³. The height is 21 mm, the width is 9 mm, and we need to find the length (denoted as $l$). 2. **Formula for volume of a cuboid:** $$\text{Volume} = \text{length} \times \text{width} \times \text{height}$$ 3. **Substitute the known values:** $$2457 = l \times 9 \times 21$$ 4. **Simplify the multiplication of width and height:** $$9 \times 21 = 189$$ So, $$2457 = l \times 189$$ 5. **Solve for length $l$ by dividing both sides by 189:** $$l = \frac{2457}{189}$$ 6. **Calculate the division:** $$l = 13$$ 7. **Answer with correct units:** The length of the cuboid is **13 mm**.