1. **State the problem:** We have a rectangular prism with a square base, volume $3600$ m³, and height $20$ m. We need to find the length of one side of the square base. 2. **Formula:** The volume $V$ of a rectangular prism is given by: $$V = \text{base area} \times \text{height}$$ Since the base is square with side length $s$, the base area is $s^2$. So: $$V = s^2 \times h$$ 3. **Substitute known values:** $$3600 = s^2 \times 20$$ 4. **Solve for $s^2$:** $$s^2 = \frac{3600}{20}$$ 5. **Simplify the fraction:** $$s^2 = \frac{\cancel{3600}}{\cancel{20}} = 180$$ 6. **Find $s$ by taking the square root:** $$s = \sqrt{180}$$ 7. **Simplify the square root:** $$\sqrt{180} = \sqrt{36 \times 5} = \sqrt{36} \times \sqrt{5} = 6\sqrt{5}$$ 8. **Approximate the value:** $$6 \times 2.236 = 13.416$$ **Final answer:** The length of one side of the base is $6\sqrt{5} \approx 13.42$ meters.