jmc

The greatest common factor of $28z^{97}$ and $7z^{96}$ is $7z^{96}$. We factor $7z^{96}$ out of both terms to get$$\begin{array}{rl}28z^{97}+7z^{96}& =7z^{96}\cdot 4z+7z^{96}\cdot 1\\ & =\boxed{7z^{96}(4z+1)}.\end{array}$$