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given a-b = (1,2,3) i.e.,
        a=(1,2,3) and
         b-a= (6,7,8,9)
         i.e., b=(6,7,8,9)

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Since A and B are dis-joint sets so they have no element in common 

so A - B = {1,2,3} which gives A = {1,2,3}

and B - A = {6,7,8,9}  which gives B = {6,7,8,9}