1. **State the problem:** We need to find the area of a trapezoid with bases of lengths 12 km and 4 km, and a height of 3 km. 2. **Formula for the area of a trapezoid:** $$\text{Area} = \frac{(\text{Base}_1 + \text{Base}_2)}{2} \times \text{Height}$$ 3. **Identify the values:** - Base 1 ($b_1$) = 12 km - Base 2 ($b_2$) = 4 km - Height ($h$) = 3 km 4. **Substitute the values into the formula:** $$\text{Area} = \frac{(12 + 4)}{2} \times 3$$ 5. **Simplify inside the parentheses:** $$\text{Area} = \frac{16}{2} \times 3$$ 6. **Divide 16 by 2:** $$\text{Area} = 8 \times 3$$ 7. **Multiply:** $$\text{Area} = 24$$ 8. **Conclusion:** The area of the trapezoid is 24 square kilometers.