- \[3x(x^3-4x+6)=3x^4-12x^2+18x;\]
- \[(x-3)(2x+1)=2x^2+x-6x-3=2x^2-5x-3;\]
- \[(4a-7b)(5a+6b)=20a^2+24ab-35ab-42b^2=20a^2-11ab-42b^2;\]
- \[(y+2)(y^2+y-8)=y^3+y^2-8y+2y^2+2y-16=y^3+3y^2-6y-16.\]
Ответ: 1) \(3x^4-12x^2+18x\); 2) \(2x^2-5x-3\); 3) \(20a^2-11ab-42b^2\); 4) \(y^3+3y^2-6y-16\).