On the irreducibility of $x^3+x^2+x+2$ in $\mathbb{Z}/(n)[x]$

Question

For which $n\in \{2,3,7\}$ is the polynomial $x^3+x^2+x+2$ irreducible in $\mathbb{Z}/(n)[x]$?

My work:

For a given ring $R$ and a polynomial $p(x)\in R[x]$, if $p(\alpha)=0$ for some $\alpha\in R$, then we can conclude that $p(x)$ is reducible. But if not we cannot say that $p(x)$ is irreducible. So how do we conclude that whether it is irreducible or not in such case? In the question above, if we take $n=3$ or $7$, then we have such situation.

Answer 1

If our polynomial is of degree $2$ or $3$ over a field, it is irreducible if and only if it has no zero in the field.