The given expression is a difference of squares, so we have
16 (a + b)² - 49 (a - b)² = 4² (a + b)² - 7² (a - b)²
… = (4 (a + b))² - (7 (a - b))²
… = (4 (a + b) - 7 (a - b)) (4 (a + b) + 7 (a - b))
… = (4a + 4b - 7a + 7b) (4a + 4b + 7a - 7b)
… = (-3a + 11b) (11a - 3b)
