Attempt: Let $(R,+,*)$ be an integral domain. A non-zero, non-unit polynomial $f(x)\in R[x]$ is called irreducible polynomial over $R$ if $f(x) = g(x)h(x)$, $g(x)\in R[x]$ and $h(x)\in R[x]$ then either $g(x)$ is unit or $h(x)$ is unit.
So we can write $3x^3 + 3x$ as $(3x)(x^2+1)$ with $x=0$ we have $(3x)$ is a unit, so its irreducible over $Z[x]$ an $Q[x]$?