3f+2f-7f=（3+2-7）f=-2f
x-f+5x-4f=（1+5）x+（-1-4）f=6x-5f
2a+3b+6a+9b-8a+12b=（2+6-8）a+（3+9+12）b=24b
3pq+7pq+4pq+pq=(3+7+4+1)pq=15pq
30a²b+2b²c-15a²b-4b²c=(30-15)a²b+(2-4)b²c=15a²b-2b²c
7xy-8wx+5xy-12xy=(7+5-12)xy-8wx=-8wx