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

The actual problem is:

and I have to solve this by grouping.Thanks!

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Kevin S. |
5.0 5.0 (4 lesson ratings) (4)

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)