  The main goal of this paper is to provide a complete answer to the Problem 22
of Narkiewicz \cite{Nar} for any sextic number field $K$ generated by a complex
root $\alpha$ of a monic irreducible trinomial $F(x) = x^6+ax^5+b \in
\mathbb{Z}[x]$. Namely we calculate the index of the field $K$. In particular,
if $i(K)\neq 1$, then $K$ is not mongenic. Finally, we illustrate our results
by some computational examples.
