What would be the radius of a circle with a diameter of 8 inches?
In looking at your questions, they all could be solved pretty easily by looking in your text and checking out the formulas to solve those problems.
For example, the answer to this question is that the radius is half of the diameter, or d/2.
The answer to your question about the volume of a cube is that volume is width x length x height. A cube means that all sides are equal, therefore, the volume of a cube where the width, length, and height are all equal, such as 3, then the answer is just 3 x 3 x 3 = 27. But again, I'm sure that the formula for a cube or volume is given in the text.