You may have correctly started with $\displaystyle{3}=\sqrt{{\frac{{{3}{V}}}{{{3.14}\cdot{6}}}}}\usepackage{cmbright}\Huge\$ , simplified to $\displaystyle{3}=\sqrt{{\frac{{{3}{V}}}{{18.84}}}}\usepackage{cmbright}\Huge\$ , and then multiplied by 18.84. This will not produce $\displaystyle\sqrt{{{3}{V}}}\usepackage{cmbright}\Huge\$ on the right side, though. Square both sides before multiplying or dividing to isolate V.