This can be written as (27 x^6)^2/3=((27 x^6)^1/3)^2)
x^1/n means nth power of x
Therfore (27x^6)^1/3=cube root of 27x^6
=3x^2
Therefore (27x^6)^2/3=(3x^2)^2
=9x^4
Your initial line is right: 27^(2/3)*x^6(2/3) but 27^2/3 is not done by multiplying it but rather it means square of the cube root of 27 or getting first cube root of 27 is 3 then squaring it gives you 9.