13.4
1)(b+3)(b²-b+7)=b³-b²+7b+3b²-3b+21=b³+2b²+4b+21
2)(2-a)(16-a+a²)=32-2a+2a²-16a+a²-a³=32+3a²-18a-a³
3)(a+4)(a²+a-2)=a³+a²-2a+4a²+4a-8=a³+5a²+2a-8
4)(5-b)(4-b-b²)=20-5b-5b²-4b+b²+b³=b³-4b²-9b+20
5)(3xy-4)(6+xy)=18xy+3x²y²-24-4xy=3x²y²+14xy-24
6)(4nm+3)(nm-8)=4n²m²-32nm+3nm-24=4n²m²-29nm-24