Dear

   first lets find out the potential at the center of the metalic sphere it will be because of the induced charge on the surface of the metallic conductor and due to the point charge you can comprehend that we chose the centre as the point since we want make things simple why to bother about unnessary mathematical calculation associated with any other point when we KNOW POTENTIAL AT EVERY POINT ON THE METALIC CONDUCTOR IS CONSTANT  , so after chosing this point we can see that by symmtery the induced charge on the surface will have no contribution since they are equal and oppsite and there distribution is the mirorr image of each other so the poten tial at the center of the mettalic sphere is going to be kQ/r and this also going to be at the surafce since the potential on the sphere has to be constant . ( Other wise the system would not attent electrostatic equlibrium )