∵A=8x²y-6xy²-3xy,B=7xy²-2xy+5x²y
∴C=3（A+B)
=3(8x²y-6xy²-3xy+7xy²-2xy+5x²y)
=3(13x²y+xy²-5xy)
=39x²y+3xy²-15xy
∴C-2A
=39x²y+3xy²-15xy-2(8x²y-6xy²-3xy)
=39x²y+3xy²-15xy-16x²y+12xy²+6xy
=23x²y+15xy²-9xy
或者
C-2A=3(A+B)--2A
=A-3B
=8x²y-6xy²-3xy+3(7xy²-2xy+5x²y)
=8x²y-6xy²-3xy+21xy²-6xy+15x²y
=23x²y+15xy²-9xy