To solve this problem you must apply the proccedure shown below:
1. You have that the lenght ot the board is 6 feet and forms a 60 degree angle with the ground. Therefore, you have:
Sinα=opposite/hypotenuse
α=60°
opposite=x
hypotenuse=6
2. When you substitute the values, you obtain:
Sin(60°)=x/6
3. Now, you must solve for x, as following:
x=6Sin(60°)
x=5.19
Therefore, the answer is: 5.19 feet.