Answer: 6i) 6a² - 2ab
6ii) -2b³
6iii) b³ + 7b² - 49b
<u>Step-by-step explanation:</u>
6i) (a + b)(5a - 3b) + (a - 3b)(a - b)
= 5a² - 3ab + 5ab - 3b² + a² - ab - 3ab + 3b²
= 5a² + 2ab - 3b² + a² - 4ab + 3b²
= 6a² - 2ab
6ii) (a - b)(a² + b² + ab) - (a + b)(a² + b² - ab)
= a³ + ab² + a²b - ab² - a²b - b³ - (a³ + ab² - a²b -ab² + a²b + b³)
= a³ - b³ - (a³ + b³)
= a³ - b³ - a³ - b³
= -2b³
6iii) (b² - 49)(b + 7) + 343
= b³ + 7b² - 49b - 343 + 343
= b³ + 7b² - 49b
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