3f+2f-7f=-2f
x-f+5x-4f=6x-5f
2a+3b+2b平方c-15a平方b-4b平方c
=2a+3b+2b^2c-15a^2b-4b^2c
3pq+7pq+4pq+pq
=(3+7+4+1)pq
=15pq
7xy-8wx+5xy-12xy
=(7+5-12)xy-8wx
=-8wx