a-2b
=(5x²y-3xy²+4xy)-2(7xy²-2xy+x²y)
=5x²y-3xy²+4xy-14xy²+4xy-2x²y
=3x²y+8xy-17xy²
-2分之1﹙a+b﹚-a
=-1/2a-1/2b-a
=-3/2a-1/2