Given,

9(2a – b)² –4 (2a – b) – 13

Let us assume (2a – b) = x

9x² – 4x – 13

9x² – 13x + 9x – 13

x(9x – 13)+1 (9x + 3)

(9x – 13) (x + 1)

[9(2a – b) – 13] [2a – b + 1]

(18a – 9b – 13) (2a – b + 1)