5. ($$\frac{x}{9z}-\frac{z}{x}$$) : $$\frac{x-3z}{18xz}$$ = $$\frac{x^2-9z^2}{9xz}$$ : $$\frac{x-3z}{18xz}$$ = $$\frac{(x-3z)(x+3z)18xz}{9xz(x-3z)}$$ = 2(x+3z) = 2x+6z
Ответ: 2x+6z