kindly solve it by giving explanation.......

If a+b = 12, and 1/a + 1/b= 6, then what should be the value of ab


(A) 1/2
(B) -3
(C) 2
(D) -2
(E) 1/6

I have seen this problem on www.TC.com/GRE, in sample questions. kindly solve it by an another method

Looking for a good reply.