Factor completely:

54+16x^3

54 + 16x^3

2(8x^3 + 27)
then recall the how we factor sum of two cubes:
a^3 + b^3 = (a+b)(a^2 - ab + b^2)
therefore,
2(2x+3)(4x^2 - 6x + 9)

hope this helps~ :)