12.(a-b+c)(-a+b-c)=[a-(b-c)][b+(-a-c)]
13.A=m²-2n² B=1+n²+2m²
2A-3B=2(m²-2n²)-3(1+n²+2m²)
=2m² -4n² -3-3n² -6m²
=-7n² -4m² -3
14.x²-3x²y-(3x²y-3xy²)
=x²-6x²y+3xy²
12.(a-b+c)(-a+b-c)=[a-(b-c)][b+(-a-c)]
13.A=m²-2n² B=1+n²+2m²
2A-3B=2(m²-2n²)-3(1+n²+2m²)
=2m² -4n² -3-3n² -6m²
=-7n² -4m² -3
14.x²-3x²y-(3x²y-3xy²)
=x²-6x²y+3xy²