Answer:
6 feet.
Step-by-step explanation:
Remember that the foci of the semi-ellipse are two points in the base of the semi-ellipse from where you can create the full perimeter of the semi-ellipse, basically form there you can sketch it. And the sum of both to the same spot in the perimeter always adds up to the same number.
To calculate it you just need to remember that the major axis is always equal to that rope that you use to draw the semi-ellipse and since the major axis is the same as the base the rope will be 6 feet.