$(x-7)(x^2-9x+20)(x-2)=72_{}$
$⇔(x^3-9x^2+20x-7x^2+63x-140)(x-2)=72_{}$
$⇔(x^3-16x^2+83x-140)(x-2)=72_{}$
$⇔x^4-2x^3-16x^3+32x^2+83x^2-166x-140x+280=72_{}$
$⇔x^4-18x^3+115x^2-306x+280=72_{}$
$⇔x^4-18x^3+115x^2-306x+280-72=0_{}$
$⇔x^4-x^3-17x^3+17x^2+98x^2-98x-208x+208=0_{}$
$⇔x^4.(x-1)-17x^2.(x-1)+98x.(x-1)-208.(x-1)=0_{}$
$⇔(x-1)(x^3-17x^2+98x-208)=0_{}$
$⇔(x-1)(x^3-8x^2-9x^2+72x+26x-208)=0_{}$
$⇔(x-1).[ x^2.(x-8)-9x.(x-8)+26.(x-8)]=0_{}$
$⇔(x-1)(x-8)(x^2-9x+26)=0_{}$
$(vì_{}$ $x^2-9x+26∉R)_{}$
$⇔_{}$ \(\left[ \begin{array}{l}x-1=0\\x-8=0\end{array} \right.\) \(\left[ \begin{array}{l}x=1\\x=8\end{array} \right.\)
$Vậy_{}$ $x_{1}=1;$ $x_{2}=8$
