3m^2 + 5mn + 7m^2y - 5mn - 3m^2 - 7m^2y - 3m^2 =
(3m^2 - 3m^2) + (5mn - 5mn) + (7m^2y - 7m^2y) = 0
Therefore, the answer is 0.
3m^2 + 5mn + 7m^2y - 5mn - 3m^2 - 7m^2y - 3m^2 =
(3m^2 - 3m^2) + (5mn - 5mn) + (7m^2y - 7m^2y) = 0
Therefore, the answer is 0.