This is a factor of the second monomial, but it is not a factor of the first one. For each prime or variable they have in common, count how many times they have that factor in common. The GCF should have that many. The variable y occurs three times as a factor in $\displaystyle{16}{{x}}^{{4}}{{y}}^{{3}}\usepackage{cmbright}\Huge\$ , so $\displaystyle{{y}}^{{4}}\usepackage{cmbright}\Huge\$ should not be a factor of the GCF.