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= 16a² - 25(b² - 2bc + c²)
= 16a² - 25b² + 50bc - 25c²
b) 49(2a - 3b)² - 4(3a + b)²
= 49(4a² - 12ab + 9b²) - 4(9a² + 6ab + b²)
= 196a² - 588ab + 441b² - 36a² + 24ab + 4b²
= 160a² - 564ab + 445b²