#((8x^3)/(27y^6))^(1/3)# is the same as #((color(green)(2^3)x^3)/(color(green)(3^3)y^6))^(1/3)#
Distribute the external exponent #(color(red)(1/3))#into the exponents inside (for both numerator and the denominator)
#((color(green)(2^3)x^3)/(color(green)(3^3)y^6))^color(red)(1/3)=(color(green)(2^(3*color(red)(1/3)))x^(3*color(red)(1/3)))/(color(green)(3^(3*color(red)(1/3)))y^(6*color(red)(1/3)))=(color(green)2x)/(color(green)3y^2)#