Let $G$ to be group of order 99 and has a normal subgroup of order 9. prove that $G$ is abelian and find how many non isomorphic groups are there Let G be a group such that $|G| = 99$, and let $Z(G)$ be the center of $G$.
$Z(G)$ is a normal subgroup of G and $|Z(G)|$ must be 1,3,9,11,33, or 99.
I will use that if  $G/Z(G)$ is cyclic, then $G$ is abelian.
I know that I have a normal subgroup $|N|=9 $
if I prove that $Z(G)=N$ then $|G /Z(G)| = 99/9 = 11 $
11 is prime so $G/Z(G)$ is cyclic, then $G$ is abelian
should I prove that $Z(G)=N$ or can I assume that? if yes, how to find the non isomorphic 
groups?
and if I can't assume that $Z(G)=N$, how can I prove that ?

Since you've already studied Sylow theorems I think it can be a fast proof. Let us denote by $\;n_p\;$ the number of Sylow $\;p$- subgroups, so in this case we must have
$$\begin{align}&n_{11}=1\;,\;\;\text{since this is the only natural that both divides}\;3^2=9\;\text{and is } =1\pmod{11}\\
&n_3=1\;,\;\;\text{since this is the only natural that both divides}\;11\;\text{ and is} =1\pmod 3\end{align}$$
thus, both Sylow subgroups of $\;G\;$ ( say $\;P_{11}\,,\,\,P_3\;$) are normal, abelian (why?) and also
$$P_{11}\cap P_3=\{1\}\;,\;\;G=P_{11}P_3$$
As you can see, we don't need to be given that $\;P_3\;$ is normal: it follows at once from the Sylow theorems.
From the above, it follows these Sylow subgroups generate their direct product and thus
$$G=P_{11}P_3\cong P_{11}\times P_3$$
