Ответ:

1) 3x(x²-2x+3)=3x³-6x²+9x;

2)-4a•(a²-3ab+7b)= -4a³+12a²b-28ab;

3)2,5(6x-4)+3(x-3)-8(1-4x)=15x-10+3x-9-8+32x;

4)10x(5x²-7y)-6x(5y+9x²)=50x³-70xy-30xy-54x³.