1. **State the problem:** We are given the formula for volume $V$ as $$V = \frac{1}{2} L (B + b)$$ where $B = 0.2$ m², $b = 0.5$ m², and $V = 0.21$ m³. We need to find the length $L$ in metres. 2. **Write the formula and substitute known values:** $$0.21 = \frac{1}{2} L (0.2 + 0.5)$$ 3. **Simplify inside the parentheses:** $$0.21 = \frac{1}{2} L (0.7)$$ 4. **Multiply the constants:** $$0.21 = \frac{1}{2} \times 0.7 \times L = 0.35 L$$ 5. **Solve for $L$ by dividing both sides by 0.35:** $$L = \frac{0.21}{0.35}$$ 6. **Show cancellation step:** $$L = \frac{\cancel{0.21}}{\cancel{0.35}} \times \frac{1}{1} = 0.6$$ 7. **Final answer:** $$L = 0.6 \text{ metres}$$ This means the length $L$ is 0.6 metres.