= (81/256)^(- 3/4) → you know that: 81 = 3 * 3 * 3 * 3 = 3^4
= [(3^4)/256]^(- 3/4) → you know that: 256 = 4 * 4 * 4 * 4 = 4^4
= [(3^4)/(4^4)]^(- 3/4)
= [(3/4)^4]^(- 3/4) → recall: (x^a)^b = x^(ab)
= (3/4)^[4 * (- 3/4)]
= (3/4)^(- 3) → recall: x^(- a)= 1/x^(a)
= 1/(3/4)^3
= 1/[(3^3) / (4^3)]
= (4^3) / (3^3)
= 64/27