How to prove this (basic big theta problem)

Question

How to prove that if $$f(x)=a_n x^n +a_{n-1}x^{n-1}+\cdots +a_1x+a_0$$ and $a_n\not=0,a_0,a_1,\ldots,a_{n-1}$ are real numbers, then $f(x)=\Theta(x^n)$.