In the image created, we can see that we can apply sin ∅ = opposite / hypotenuse. the opposite side is the distance between 2 parallel bars and the ∅ is the angle made by the diagonal on top.
Given:
∅ = 60°
Opposite = 9 ft
Hypotenuse = 9 / sin 60 = 10.39 ft
the length of the support AB is 10.39 ft