`64^x : 16^x = 256`

`( 2^6 )^x : ( 2^4 )^x = 2^8`

`2^(6x) : 2^(4x) = 2^8`

`2^( 6x - 4x) = 2^8`

`2^(2x) = 2^8`

`2x = 8`

`x = 8 : 2`

`x = 4`

`------`

`( a^n )^m = a^(n . m)`