Evaluate:
$(a - 3 b)^{2} - 4 (a - 3 b) - 21$

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Solution

$(a - 3 b)^{2} - 4 (a - 3 b) - 21 L e t a - 3 b = x x^{2} - 4 x - 21 = x^{2} - 7 x + 3 x - 21 = x (x - 7) + 3 (x - 7) = (x - 7) (x + 3) = (a - 3 b - 7) (a - 3 b + 3)$

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