Answer:
9x² - 49
Explanation:
A difference of squares factors in general as
a² - b² = (a + b)(a - b)
Given (3x + 7) with a = 3x and b = 7
Then the other factor is (3x - 7)
(3x + 7)(3x - 7)
= (3x)² - 7²
= 9x² - 49
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