We know that,

a³ + b³ = (a + b) (a² – a × b + b²)

Using this formula, we get

= 7 (a³ + 8b³)

= 7 (a + 2b) (a² – a × 2b + (2b)²]

= 7 (a + 2b) (a² – 2ab + 4b²)