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Let, +q and –q are the charges along the line               
Potential at a point, which is at a distance r from the center,

           1              q
 V = -- ------- X -------                                                                                                                       4 πε0          r     
                              1              q

So, here Vq    = ------- X ----------                                  
                             4 πε0    (a + r)

             1              -q
V-q    = ------- X ----------
           4 πε0         (r-a)

Now,  V = Vq + V-q        
                                      q            2a 
So,       Potential   = -------  X  ----------                          
                                  4 πε0      (a2-r2)