（1）7m-3n^2+9m+3n^2
=7m+9m+3n^2 -3n^2
=16m
（2）5a^2b-2-3ab^2-7a^2b+ab^2+2a^2b-4
=5a^2b-7a^2b+2a^2b+ab^2+2a^2b-3ab^2-4-2
=(5-7+2)a^2b+(1+2-3)ab^2-6
=-6