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

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