So, I was doing an algebra exercise related to $\gcd$'s and $PID$'s and I need to check if some polynomials in $\mathbb{Z}_7[x]$ are units. The polynomials are the following:

\begin{equation*} g = x^2+5 \hspace{.5cm} \wedge \hspace{.5cm} h = x^2+6 \end{equation*} I know that if $g$ is a unit it must be invertible, i.e. \begin{equation*} \exists f \in \mathbb{Z}_7[x] \hspace{.15cm} | \hspace{.15cm}fg = 1 \end{equation*} but I can't seem to get one for any of the polynomials $f$ and $g$. Any help would be apreciatted.