Answer:
(7x-3) (3x^2 -4)
Step-by-step explanation:
21x^3 - 9x^2 - 28x + 12
We can factor by grouping
Factor out 3x^2 from the first two terms and -4 from the last two
21x^3 - 9x^2 - 28x + 12
3x^2 ( 7x-3) -4(7x-3)
Now factor out (7x-3)
(7x-3) (3x^2 -4)
