$$x^{18} = x^{9 \cdot 2} = (x^9)^2$$
$$x^{18} = x^{6 \cdot 3} = (x^6)^3$$
$$x^{18} = x^{3 \cdot 6} = (x^3)^6$$
Ответ: $$(x^9)^2, (x^6)^3, (x^3)^6$$