7xy-8wx+5xy-12xy
=(7xy+5xy-12xy)-8wxx
=0-8wx
=-8wx
2a+3b+6a+9b-8a+12b
=(2a+6a-8a)+(3b+9b+12b)
=0+24b
=24b
30a²b+2b²c-15a²b-4b²c
=(30a²b-15a²b)+(2b²c-4b²c)
=15a²b-2b²c