Given,
(2a + b)2 - 6a - 3b - 4
= (2a + b)2 - 3(2a + b) - 4
Substituting (2a + b) = x, we get :
= x2 - 3x - 4
= x2 - 4x + x - 4
= x(x - 4) + 1(x - 4)
= (x - 4)(x + 1)
= (2a + b - 4)(2a + b + 1).
Hence, (2a + b)2 - 6a - 3b - 4 = (2a + b - 4)(2a + b + 1).