(256^1/4)^3 (6^-1)
First, use this exponent rule:
(a^b)^c = a^bc
Second, you want to use another exponent rule to make the second term positive:
a^-1 = 1/a
(256^1/4)^3 (6^-1)
(256^(1/4)(3) x (1/6)
(64) (1/6)
64/6 —-simplify by 2
32/3
I hope this helps.