You may have mistaken $\displaystyle{\sqrt[{{3}}]{{{8}{x}}}}\usepackage{cmbright}\Huge\$ for $\displaystyle{8}{\sqrt[{{3}}]{{{x}}}}\usepackage{cmbright}\Huge\$ , and then added the first and third terms. Start by simplifying the second term using $\displaystyle{\sqrt[{{x}}]{{{a}{b}}}}={\sqrt[{{x}}]{{{a}}}}\cdot{\sqrt[{{x}}]{{{b}}}}\usepackage{cmbright}\Huge\$ .