To calculate the time it takes for the two zebras to walk to the Sunglasses R Us store, we can use the formula:
\[ \text{Time} = \frac{\text{Distance}}{\text{Speed}} \]
In this case, the distance is 500 meters and the speed is 2.1 m/s.
Substituting the values into the formula:
\[ \text{Time} = \frac{500 \text{ m}}{2.1 \text{ m/s}} \approx 238.1 \text{ seconds} \]
Thus, it would take the zebras approximately 238.1 seconds to reach the store.
Note: If "df" in your question refers to "distance to final" or any specific terminology, please clarify so I can assist you further!