The greatest common factor in this expression is 3x^4. Let's take it out:
3x^4(u^4 - 1)
u^4 - 1 is a difference of two squares and can be factored further:
3x^4(u^2 + 1)(u^2 - 1)
u^2 - 1 is also a difference of two squares:
3x^4(u^2 + 1)(u + 1)(u - 1)
