You may have incorrectly included the negative sign outside the radical when setting up the inequality, mistakenly using $\displaystyle-{{x}}^{{2}}-{4}\ge{0}\usepackage{cmbright}\Huge\$ to find the domain. The domain is only restricted by the value under the radical; you should be using $\displaystyle{{x}}^{{2}}+{4}\ge{0}\usepackage{cmbright}\Huge\$ to find the domain.