Answer: 21a² - 40b² - 32c² - 11ab + 72bc + 4ac

Step-by-step explanation:

(3a - 5b +4c)(7a + 8b - 8c)

= (3a × 7a) + (3a × 8b) + (3a × -8c) + (-5b × 7a) + (-5b × 8b) + (-5b × -8c) +  (4c × 7a) + (4c × 8b) + (4c × -8c)

= 21a² + 24ab - 24ac - 35ab - 40b² + 40bc + 28ac + 32bc - 32c²

= 21a² - 40b² - 32c² - 11ab + 72bc + 4ac