3m^2 + 5mn + 7m^2y - 5mn - 3m^2 - 7m^2y - 3m^2 =
3m2−3m23m^2 - 3m^23m2−3m2 + 5mn−5mn5mn - 5mn5mn−5mn + 7m2y−7m2y7m^2y - 7m^2y7m2y−7m2y = 0
Therefore, the answer is 0.
3m^2 + 5mn + 7m^2y - 5mn - 3m^2 - 7m^2y - 3m^2 =
3m2−3m23m^2 - 3m^23m2−3m2 + 5mn−5mn5mn - 5mn5mn−5mn + 7m2y−7m2y7m^2y - 7m^2y7m2y−7m2y = 0
Therefore, the answer is 0.