How can I solve this equation by factoring polynomials by grouping?

Question

The actual problem is:

and I have to solve this by grouping.Thanks!

Answer 1

21x³ + 6x² -28x - 8

It looks like you can pull a 7x out of the first and third terms, and a 2 from the second and fourth:

7x(3x² - 4) + 2(3x² - 4)

Then factor out what's in the parentheses to get (3x² - 4)(7x + 2)