$$\lim\limits_{x\to\infty}(x^n\cdot e^{-x})\\ =\lim\limits_{x\to\infty}(x^n)\cdot \lim\limits_{x\to\infty}(e^{-x})\\ =\lim\limits_{x\to\infty}(x^n)\cdot \frac{1}{e^\infty}\\ \lim\limits_{x\to\infty}(x^n)\cdot\frac{1}{\infty}\\ =\lim\limits_{x\to\infty}(x^n)\cdot 0 = 0$$