a: \(-\left(x-2y\right)\left(6x^2+3xy+x\right)\)
\(=-\left(6x^3+3x^2y+x^2-12x^2y-6xy^2-2xy\right)\)
\(=-6x^3+9x^2y-x^2+6xy^2+2xy\)
b: \(-\dfrac{18x^4y^3-24x^3y^4-12x^3y^3}{6x^2y^3}\)
\(=-\dfrac{6x^2y^3\cdot3x^2-6x^2y^3\cdot4xy-6x^2y^3\cdot2x}{6x^2y^3}\)
\(=-\dfrac{6x^2y^3\left(3x^2-4xy-2x\right)}{6x^2y^3}=-\left(3x^2-4xy-2x\right)\)
\(=-3x^2+4xy+2x\)
`-(x-2y)(3xy+6x^2+x)`
`=-(3x^2 y + 6x^3 +x^2 -6xy^2 - 12x^2y -2xy)`
`=-( 6x^3 +x^2 -2xy-9x^2 y -6xy^2)`
`= -6x^3 -x^2 +2xy +9x^2y+6xy^2`
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`-(18x^4y^3 -24x^3y^4 -12x^3y^3):(6x^2y^3)`
`= - ( 3x^2- 4xy - 2x)`
`= -3x^2+4xy+2x`
