Note that [tex]\sqrt[2]{x}=x^{\frac{1}{2}}[/tex]
[tex]\sqrt[2]{16x^{36}}=\sqrt[2]{4^2x^{18\cdot2}}[/tex]
[tex]4^{2\cdot\frac{1}{2}}x^{18\cdot2\cdot\frac{1}{2}}=4^{\frac{2}{2}}x^{\frac{18\cdot2}{2}}[/tex]
[tex]\boxed{4x^{18}}[/tex]
Hope this helps.
r3t40