(x+y)(x-2y)-(-x-2y)(-x+2y)
=x²-2xy+xy-2y²-(x²-4y²)
=x²-xy-2y²-x²+4y²
=-xy+2y²
(-a+b)²-2(a+2b)(2b-a)
=(a²-2ab+b²)-2(4b²-a²)
=a²-2ab+b²-8b²+2a²
=3a²-2ab-7b²
(m-2)(m³+4)(m+2)
=(m²-4)(m²+4)
=m^4-16
(m-2n+3p)(m+2n+3p)
=[(m+3p)-2n][(m+3p)+2n]
=(m+3p)²-4n²
=m²+6mp+9p²-4n²