If 3a - b = 1 and ab = 4, then the value of $(9a^2 - b^2)$ is :

7 8 5 9

7

If 3a - b = 1 ab = 4, then the value of $(9a^2 - b^2)$ If x - y  = n then, x + y  = \(\sqrt {n^2 + 4xy}\) and we also know that, a2 - b2 = (a + b) (a – b) Then, 3a + b = \(\sqrt {1^2 + 4×4×3}\) = 7 then the value of $(9a^2 - b^2)$ = (3a + b)(3a - b) = 7 × 1 = 7