From Förberedande kurs i matematik 1

Both terms contain x, which can therefore be taken out as a factor (as can 2),

\displaystyle 18x-2x^3=2x\cdot 9-2x \cdot x^2=2x(9-x^2)\,\textrm{.}

The remaining second-degree factor \displaystyle 9-x^2 can then be factorized using the conjugate rule

\displaystyle 2x(9-x^2)=2x(3^2-x^2)=2x(3+x)(3-x)\,,

which can also be written as \displaystyle -2x(x+3)(x-3).