If a + 3b = 12 and ab = 9, then the value of (a - 3b) is:

If a + 3b = 12

ab = 9,

then the value of (a - 3b)= ?

If x + y  = n

then, x - y  = \(\sqrt {n^2 - 4xy}\)

= (a - 3b) = \(\sqrt {12^2 - 4(3)(9)}\)

= a - 3b = \(\sqrt {144 - 108}\)

a - 3b = \(\sqrt {36}\) = 6