Question

if a=3+b then what is the value of a³-b³-9ab

Answer 1

See the attach file for ans

Answer 2

Given,
a = 3 + b
⇒ a - b = 3 -------------------------------------(i)
Take cube to both side of the eqation
⇒ (a - b)³ = 3³ = 27
⇒ a³ - b³ - 3ab(a - b) = 27
⇒ a³ - b³ - 3ab(3)  = 27        ∵a-b = 3 
⇒ a³ - b³ - 9ab = 27