Not sure if I'm right but I think it's 3(x - 6) (x^2 + 5x)
Explanation:
3x^3 - 3x^2 - 90x
Apply GCF: 3 (x^3 - x^2 - 30)
Split 30 into -6 and 5
(x^3 - 6x^2) (5x^2 - 30x)
GCF of both: x^2 (x - 6) and 5x (x - 6)
DON'T FORGET TO CARRY THE 3
And your answer is 3 (x - 6) (x^2 + 5x)