III)2.
[tex]\it \dfrac{21}{3n-5\inZ} \Rightarrow 3n-5|21 \Rightarrow 3n-5\in D_{21} \Rightarrow 3n-5\in\{\pm1,\ \pm3,\ \pm7,\ \pm21\} \Rightarrow \\ \\ \Rightarrow 3n-5\in\{-21,\ -7,\ -3,\ -1,\ 1,\ 3,\ 7,\ 21\}|_{+5} \Rightarrow\\ \\ \Rightarrow 3n\in\{-16,\ -2,\ 2,\ 4,\ 6,\ 8,\ 12,\ 26\} \stackrel{n\in\mathbb{Z}}{\Longrightarrow}\ 3n\in\{6,\ \ 12\}|_{:3} \Rightarrow n\in\{2,\ \ 4\}[/tex]
