You may have mistakenly thought $\displaystyle\sqrt{{{4}{x}}}\usepackage{cmbright}\Huge\$ was $\displaystyle{4}\sqrt{{x}}\usepackage{cmbright}\Huge\$ , and then added the first and third terms. Start by simplifying the last term using $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{a}}\cdot\sqrt{{b}}\usepackage{cmbright}\Huge\$ . Now the first and third terms are like terms that you can combine, just as you would combine $\displaystyle{9}{a}+{2}{a}\usepackage{cmbright}\Huge\$ .