If x + 1/16x = 3, then 16x3 + 1/(256x^3) is equals to:

Given, x + \(\frac{1}{16x}\) = 3

multiply by 4

so, 4x + \(\frac{1}{4x}\) = 12

64x³ + \(\frac{1}{64x^3}\) = 1728 - (3 × 12) = 1692

Divide by 4

16x³ + \(\frac{1}{256x^3}\) = \(\frac{1692}{4}\) = 423