The answer is 3√2 ft.
We will use the Pythagorean theorem to calculate this. In the right triangle, diagonal, d, is hypotenuse while sides, s, of the net are sides of the triangle:
d² = s² + s²
d² = 2s²
d = 6 ft
6² = 2s²
36 = 2s²
s² = 36/2
s² = 18
s = √18
s = √(9 * 2)
s = √9 * √2
s = 3√2 ft