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:
$=(3x)^3+3(3x)^2(4)+3(3x)(4^2)+4^3
\\=27x^3+12(9x^2)+9x(16)+64
\\=27x^3+108x^2+144x+64$