Final answer to the problem
$3x^{4}\left(2x-7\right)$
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Step-by-step Solution
Specify the solving method
1
Combining like terms $-9x^3$ and $-12x^3$
$\left(6x^4-21x^3\right)x$
2
Factor the polynomial $\left(6x^4-21x^3\right)$ by it's greatest common factor (GCF): $3x^{3}$
$3x^{4}\left(2x-7\right)$
Final answer to the problem
$3x^{4}\left(2x-7\right)$