Let's go through the conversions step by step:

1. 2.18 dm² to cm²: $1 \text{ dm²} = 100 \text{ cm²}$ $2.18 \text{ dm²} = 2.18 \times 100 = 218 \text{ cm²}$

2. 500 mm² to cm²: $1 \text{ cm²} = 100 \text{ mm²}$ $500 \text{ mm²} = \frac{500}{100} = 5 \text{ cm²}$

3. 400 cm² to dm²: $1 \text{ dm²} = 100 \text{ cm²}$ $400 \text{ cm²} = \frac{400}{100} = 4 \text{ dm²}$

4. 5.15 hm² to hectares (ha): $1 \text{ hm²} = 1 \text{ hectare (ha)}$ $5.15 \text{ hm²} = 5.15 \text{ ha}$

5. 7.4 dm² to m²: $1 \text{ m²} = 100 \text{ dm²}$ $7.4 \text{ dm²} = \frac{7.4}{100} = 0.074 \text{ m²}$

6. 350,000 m² to km²: $1 \text{ km²} = 1,000,000 \text{ m²}$ $350,000 \text{ m²} = \frac{350,000}{1,000,000} = 0.35 \text{ km²}$

7. 1,400 m² to cm²: $1 \text{ m²} = 10,000 \text{ cm²}$ $1,400 \text{ m²} = 1,400 \times 10,000 = 14,000,000 \text{ cm²}$

8. 0.004 hm² to hectares (ha): $1 \text{ hm²} = 1 \text{ hectare}$ $0.004 \text{ hm²} = 0.004 \text{ ha}$

Do you have any questions or need more details on the conversions?

Here are five related questions to expand on these ideas:

1. How do you convert from square meters (m²) to square millimeters (mm²)?
2. What is the formula for converting square kilometers (km²) to square meters (m²)?
3. How do you convert between hectares (ha) and square kilometers (km²)?
4. What is the difference between converting between area units and length units?
5. How would you convert from square decimeters (dm²) to square kilometers (km²)?

Tip: Always remember to square the conversion factor when converting between area units (e.g., $1 \text{ m} = 100 \text{ cm}$, so $1 \text{ m²} = 10,000 \text{ cm²}$).