Answer
$27x^3-108x^2+144x-64$
Work Step by Step
RECALL:
$(a-b)^3=a^3-3a^2b+ 3ab^2-b^3$
Use the formula above with $a=3x$ and $b=4$ to obtain:
$=(3x)^3-3(3x)^2(4)+3(3x)(4^2)-4^3
\\=27x^3-12(9x^2)+9x(16)-64
\\=27x^3-108x^2+144x-64$