в) $$ \frac{x^{2n}x^{n+3}}{x^{3n}} = \frac{x^{2n+n+3}}{x^{3n}} = \frac{x^{3n+3}}{x^{3n}} = x^{3n+3-3n} = x^3 $$
Ответ: $$x^3$$