Problem with Irreductible Polynomials in $k[x]$, $k$ be an field.

Question

I don't have idea, how i can prove this: Let $k$ be a field, and let $f(x) = a_{0} + a_{1}x + \cdots + a_{n}x^{n} \in k[x]$ have degree $n$ and nonzero constant term $a_0$. Prove that if $f(x)$ is irreducible, then so is $a_{n} + a_{n−1}x +\cdots+a_{0}x^{n}$.