\(2,\\ a,=\left(2x-1\right)\left(x^2+9\right)=2x^3-x^2+18x-9\\ b,=7x^2-28x-14x^3+x^2-25x+12=-14x^3+8x^2-53x+12\\ c,=8x+12-10x^2-15x=-10x^2-7x+12\)
\(3,\\ a,=3x^2+12x-7x+20+2x^3-3x^2-2x^3-5x=20\\ b,=6x-3-5x+15+18x-24-19x=-12\)
\(4,\\ a,\Leftrightarrow3x+10-2x=0\Leftrightarrow x=-10\\ b,\Leftrightarrow2x^3+9x^2-5x-2x^3-9x^2+x+\dfrac{9}{2}=\dfrac{7}{2}\\ \Leftrightarrow-4x=-1\Leftrightarrow x=\dfrac{1}{4}\)
\(5,\\ a,=20x^3-12x^2y-20x^3-5x^2y=-17x^2y=-17\cdot4\left(-3\right)=-204\\ b,=x^2-6x+8-x^2+4x-3=-2x+5=-2\cdot\dfrac{7}{4}+5=\dfrac{3}{2}\)