1. **State the problem:** Find the area of a trapezoid with top base $5$ mi, bottom base $3$ mi, and height $6$ mi. 2. **Formula for the area of a trapezoid:** $$\text{Area} = \frac{(\text{base}_1 + \text{base}_2)}{2} \times \text{height}$$ 3. **Substitute the given values:** $$\text{Area} = \frac{(5 + 3)}{2} \times 6$$ 4. **Simplify inside the parentheses:** $$\text{Area} = \frac{8}{2} \times 6$$ 5. **Simplify the fraction:** $$\text{Area} = \cancel{\frac{8}{2}}^{4} \times 6$$ 6. **Multiply:** $$\text{Area} = 4 \times 6 = 24$$ 7. **Final answer:** The area of the trapezoid is **24 square miles**.