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21x² - 7x - 28

Explanation:
To answer this question, we will simply multiply each term from the first bracket by each term from the second bracket and then combine like terms to get the final expression.

This can be done as follows:
(3x - 4)(7x + 7)
3x(7x) + 3x(7) - 4(7x) - 4(7)
21x² + 21x - 28x - 28
21x² - 7x - 28

Hope this helps :)

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