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Kc question??

Hey gues so Ive got a question I desparately need help with...

N2+3H2--->2NH3

the chemist mixes 9 mol of N2 with 27 mol of H2. the mixture is left to reach equilibrium at constant temperature.

the chemistr analyses the equilibrium mixture and finds that two thirds of the N2 and H2 had converted to ammonia. the total volume of the equilibrium mixture is 10dm3.

so far I have got:

kc=[NH3]2 / [N2] [3H2]3

and also that initially there is 9 mol of N2 27 mol of H2 and 0 mol of NH3.
then at equilibrium there is 6 mol of N2 (2/3 of 9) and 18 mol of N2 (2/3 of 27) and 24 mol of NH3. But i am getting the wrong answer and I dont know where I am going wrong. please someone help its driving me crazy XD