3A+B-[B-½(2A+4B)]
=3A+B-(B-A-2B)
=3A+B-B+A+2B
=4A+2B
=4(x³-2xy²+1)+2(x³+xy²-3x²y)
=4x³-8xy²+4+2x³+2xy²-6x²y
=6x³-6xy²-6x²y+4
=6×(-2)³-6×(-2)×(1/2)²-6×(-2)²×(1/2)+4
=-48+6-12+4
=-50