Just plugin the numbers you have. For instance, r = radius. We know that the radius is 1/2 of the diameter. Our diameter is 12 so 12 / 2 = 6. With that said, add 6 to where the r is located in the formula and solve the formula.
Solve for V
To solve for V we have to do the following.
The formula above reads V(Volume of air) equals 4 divided by 3 times pi times 6 to the power of 3