We have,

63a² – 112b²

= 7(9a² – 16b²)

By using the formula a² – b² = (a + b)(a – b)

We get,

63a² – 112b² = 7(9a² – 16b²)

= 7{(3a)² – (4b)²}

= 7(3a + 4b)(3a – 4b)