(Seemingly Complicated) Reverse Chemical Equilibria

I've had to do a homework on Chemical equilibria and could do them all except for the one below. I just ended up going into rather complicated maths quadratic equations which then gave the wrong answer. I was wondering if anyone would know an efficient way of getting the answer. Any help greatly appreciated.

CO and Cl2 react to form COCl2. A mixture was prepared containing 0.2mol of CO and 0.1mol of Cl2 in a 3dm^3 vessel. Kc = 0.410
Calculate COCl2 at equilibrium.

OK woolly non-maths way...

the equilibrium constant is low therefore you assume that there is very little product formed and keep the initial concentrations of reactants unchanged.

let moles of COCl₂ at equilibrium be z

concentration of COCl₂ = z/3
concentration of CO = 0.2/3
concentration of Cl₂ = 0.1/3

now Kc = 3z / (0.1 x 0.2) = 0.41

z = [0.41 x 0.02]/3 = 0.00273 moles = 0.00091 moles per litre