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

4 8 6 9

6

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