You forgot to take the absolute value of $\displaystyle{{s}}^{{7}}\usepackage{cmbright}\Huge\$ and $\displaystyle{{t}}^{{3}}\usepackage{cmbright}\Huge\$ . Because the variables have odd exponents, the expressions $\displaystyle{{s}}^{{7}}\usepackage{cmbright}\Huge\$ and $\displaystyle{{t}}^{{3}}\usepackage{cmbright}\Huge\$ could have a positive or negative value. You need to write $\displaystyle\sqrt{{{{\left({{s}}^{{7}}\right)}}^{{2}}}}={\left|{{s}}^{{7}}\right|}\usepackage{cmbright}\Huge\$ and $\displaystyle\sqrt{{{{\left({{t}}^{{3}}\right)}}^{{2}}}}={\left|{{t}}^{{3}}\right|}\usepackage{cmbright}\Huge\$ because the square root symbol means take the positive root.