1. **State the problem:** Find the area of a trapezoid with top base $5$ km, bottom base $9$ km, and height $3$ km. 2. **Formula for the area of a trapezoid:** $$\text{Area} = \frac{(\text{Base}_1 + \text{Base}_2)}{2} \times \text{Height}$$ 3. **Apply the values:** $$\text{Area} = \frac{(5 + 9)}{2} \times 3$$ 4. **Simplify inside the parentheses:** $$\text{Area} = \frac{14}{2} \times 3$$ 5. **Cancel common factors:** $$\text{Area} = \cancel{\frac{14}{2}} \times 3 = 7 \times 3$$ 6. **Calculate the product:** $$\text{Area} = 21$$ 7. **Final answer:** The area of the trapezoid is **21 square kilometers**.