$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
7.61
\begin{equation} \begin{gathered}\label{equation:transitivity-pullback-square} \xymatrix@R =50pt@C=50pt{ X' \ar [r] \ar [d] & Y' \ar [r] \ar [d] & Z' \ar [d] \\ X \ar [r] & Y \ar [r] & Z. } \end{gathered} \end{equation}