You forgot to take the absolute values of $\displaystyle{{x}}^{{3}}\usepackage{cmbright}\Huge\$ . Because the variable has an odd exponent, the expression $\displaystyle{{x}}^{{3}}\usepackage{cmbright}\Huge\$ could have a positive or negative value. You need to write $\displaystyle\sqrt{{{{\left({{x}}^{{3}}\right)}}^{{2}}}}={\left|{{x}}^{{3}}\right|}\usepackage{cmbright}\Huge\$ because the square root symbol means take the positive root.