square root of 81x to the 4th power
\({ \left( \sqrt { 81x } \right) }^{ 4 }\\ \\ ={ \left[ { \left( 81x \right) }^{ \frac { 1 }{ 2 } } \right] }^{ 4 }\\ \\ ={ \left( 81x \right) }^{ 2 }\\ \\ ={ 81 }^{ 2 }{ x }^{ 2 }\\ \\ =6561{ x }^{ 2 }\)