Consider an exact sequence of
\usepackage{amssymb}
\usepackage{tikz-cd}
\begin{document}
\Large
\begin{tikzcd}
0 \arrow[r] & M' \arrow[r, "f"] & M\arrow[r, "g"] & M'' \arrow[r] &0
\end{tikzcd}
\end{document}
Then we have that
Remark
Recall that if