We know that,

a³ + b³ = (a + b) (a² – a × b + b²)

Using this formula, we get

= (7)³ + (5b)³

= (7 + 5b) [(7)² – (7) (5b) + (5b)²]

= (7 + 5b) (49 – 35b + 25b²)