Answer: 
Step-by-step explanation:
The equation for direct variation between x and y (dependent variable) is given by :-
, where k is the proportionality constant.
Let the algebraic expression for the volume of sphere is v and for diameter is d.
The given statement : The volume of a sphere varies directly as the cube of its diameter.
Then the algebraic equation for the given statement will be :-
, where c is the proportionality constant.