Hello,

Let X be x^3, then you can go back to a polynomial of second degree, which you can derivate easily and find the value that annulates it.

->4X^2-8X+18

Derivative : 8X-8

Annulates in X=1 (negative then positive -> the initial function decreases first then increases -> it's a minimum)

<=> x^3=1

<=> x=1