To factor, you can first treat it like a single bracket and find the common factor. In this case, the common factor is 3x, so you get
3x(x² + 7x + 12)
Now you can factor the bracket normally, by finding factors of 12 that add up to make 7. The factors would be 3 and 4, so the bracket becomes (x + 3)(x + 4).
This leaves your final answer as
3x(x + 3)(x + 4)
I hope this helps!
