The poles, when joined, will form an isosceles triangle, with the triangle's base being equivalent to the base of the fly and the equal sides being equal to the length of the poles. If we bisect this triangle down the main vertex to divide its base into two equal parts, we end up with a right-angle triangle with:
Base = 16/2 = 8 feet
Height = 12 feet
Using Pythagoras theorem:
length = √(12² + 8²)
Length = 14.4 feet