Suppose the quotient group $G/C$ is cyclic and generated by element $Ca$ of $G/C$. Prove for any two elements $x,y \in G$ that $xy=yx$.
I am having trouble formulating a proof. In a preceding problem I was able to prove that for every $x \in G$ there is some integer $m$ such that $x=ca^m$ which I am guessing I can use to write $y=c'a^n$. Where would I go from here?