Two similar conducting balls having charges +q and -q are placed at a separation d from each other d from each other in air . The radius of each ball is r and the separation between their centres is d (d>>r ) . Calsulate the capacitance of the two balls system : 

 

if its at d, then you will have to consider the balls as sphere and find respective potentials at a distance of d for each ball
Here D<<r signifies the ball can be considered as point charges, since for point charge potential is same at all distance, the distance D did not matter and hence its not a part of final answer, in case d is not small then it will be a part of final answer