Answer:

So then is required at least 210 ft in order to satisfy the conditions.
Explanation:
For this case we can assume that the rectangular enclosure is on the figure attached.
The perimeter on this case is given by:

We know that the total area is
and the total area is given this formula:

Because we have two identical rectangular pieces.
And we can find the value for W like this:

And then we can find the perimeter like this:

So then is required at least 210 ft in order to satisfy the conditions.