Question

A{1,2,3,4}, B{4,5,6,7}, C{6,7,8,9} A uniun B = B uniun A

Answer 1

A union B=Bunion A

{1,2,3,4} union{4,5,6,7} = {4,5,6,7}union{1,2,3,4}
{1,2,3,4,5,6,7} = {1,2,3,4,5,6,7}
hence prooved