You may have correctly started with $\displaystyle{4}=\sqrt{{\frac{{{3}{V}}}{{{3.14}\cdot{8}}}}}\usepackage{cmbright}\Huge\$ , simplified to $\displaystyle{4}=\sqrt{{\frac{{{3}{V}}}{{25.12}}}}\usepackage{cmbright}\Huge\$ , and then multiplied by 25.12. 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.