To solve this problem you must apply the proccedure shown below:
1. You have that the 20-foot support post makes a 70° angle with the ground. Therefore, you have:
Sinα=opposite/hypotenuse
α=70°
opposite=x
hypotenuse=20
2. When you substitute the values, you obtain:
Sin(70°)=x/20
3. Now, solve for x:
x=20Sin(70°)
x=18.8
The answer is: 18.8 feet