You correctly realized that the symbol $\displaystyle\sqrt{{{3}{m}}}\usepackage{cmbright}\Huge\$ means the square root or 2nd root of 3m as in $\displaystyle{\sqrt[{{2}}]{{{3}{m}}}}\usepackage{cmbright}\Huge\$ . But then you incorrectly used the index of the radical as the exponent in the expression. The rule for converting a radical is $\displaystyle{\sqrt[{{n}}]{{{x}}}}={{x}}^{{\frac{{1}}{{n}}}}\usepackage{cmbright}\Huge\$ .