$\displaystyle{56}{{z}}^{{4}}={2}\cdot{2}\cdot{2}\cdot{7}\cdot{z}\cdot{z}\cdot{z}\cdot{z}\usepackage{cmbright}\Huge\$

$\displaystyle{72}{{z}}^{{6}}={2}\cdot{2}\cdot{2}\cdot{3}\cdot{3}\cdot{z}\cdot{z}\cdot{z}\cdot{z}\cdot{z}\cdot{z}\usepackage{cmbright}\Huge\$

GCF = $\displaystyle{2}\cdot{2}\cdot{2}\cdot{z}\cdot{z}\cdot{z}\cdot{z}={8}{{z}}^{{4}}\usepackage{cmbright}\Huge\$

$\displaystyle{56}{{z}}^{{4}}+{72}{{z}}^{{6}}={8}{{z}}^{{4}}{\left({7}\right)}+{8}{{z}}^{{4}}{\left({9}{{z}}^{{2}}\right)}={8}{{z}}^{{4}}{\left({7}+{9}{{z}}^{{2}}\right)}\usepackage{cmbright}\Huge\$