①(x-2)(x²+x-5)-3x³
=x³+x²-5x-2x²-2x+10-3x³
=-2x³-x²-7x+10
②
(2a-b)(a²-2ab+3b²)
=2a³-4a²b+6ab²-a²b+2ab²-3b³
=2a³-5a²b+8ab²-3b³