The unknown length is the hypotenuse of the right angle formed when connecting the top of the second floor and the distance from the wall from which the belt is installed. Using the trigonometric function, sine, the equation becomes, sin 60° = 12 ft / h The value of h from the equation is 13.86 ft. Thus, the answer is 13.86 ft.
It would be a sin or cos function, with a period of 365 (number of days), max and min would depend on your coordinates on Earth. Min would be around the winter solstice and max around summer solstice, this would determine the phase shift.