Part a: The radius of a circle is the distance from the center of the circle (the origin) to the outside of the circle (the perimeter). Since the longest distance (diameter) from side to side of this circle is 9 feet, the radius is half of that which is 4.5 feet.
Part b: The formula for the volume of a sphere is as you have written on your sheet there: V = (4/3)*pi*r^3. Substituting r = 4.5 into this equation gives 381.7 cubic feet.
Part c: Since the hemisphere is half of the sphere, its volume is half that of the sphere and thus by dividing the answer for Part b by 2 we get 190.9 cubic feet.
Note that if you use your value of pi as 3.14 or another rounded number your number may be extremely slightly different to these values as I have used the pi button on the calculator to give greater accuracy.