Here, coefficient of (2a – b)2 = 1, coefficient of (2a – b) = 2 and constant term = - 8
We shall now split up the coefficient of middle term i.e., 2 into two parts whose sum is 2 and product is -8 (1) = - 8
Clearly,
4 - 2 = 2 and
4 (-2) = - 8
So, we replace 2 (2a – b) = 4 (2a –b) – 2 (2a – b)
Thus, we have
(2a – b)2 + 2 (2a – b) – 8 = (2a – b)2 + 4 (2a – b) – 2 (2a – b) - 8
= (2a – b) (2a – b + 4) – 2 (2a – b + 4)
= (2a – b – 2) (2a – b + 4)