a)A=(a+b)-(a-b)+(a-c)-(a+c)

      = a + b - a + b + a - c - a - c

      = ( a - a ) + ( b + b ) + ( a - a ) + ( - c - c )

      = 0 + 2b + 0 + - 2c

      = 2b - 2c

b)B=(a+b-c)+(a-b+c)-(b+c-a)-(a-b-c)

      = a + b - c + a - b + c - b - c + a - a + b + c

      = ( a - a ) + ( b - b ) + ( c - c ) + ( b - b ) + ( c - c ) + ( a + a )

      = 0 + 0 + 0 + 0 + 0 + 2a

      = 2a