③(x-2y)(2x+4y)
=2(x-2y(x+2y)
=2x²-8y²

④-2a²(½ab+b²)-5a(a²b-ab²)
=-a²b(a+2b)-5a²b(a-b)
=a²b[-(a+2b)-5(a-b)]
=a²b(-a-2b-5a+5b)
=a²b(-6a+3b)
=-6a³b+3a²b²