To factor 9x^{2} - 625 ,
use the a^{2} - b^{2} = (a - b)(a + b),
noting that here, "a" is "3x" and "b" is "25" (25^{2} = 625), to get:
9x^{2} - 625 = (3x - 25)(3x + 25).

No more factoring is possible.

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