How large should the equilibrium constant be for the rxn: A + 2B <=> C + 2D so that all but 0.10% of A is converted into products when A and B are reacted in an initial molar ratio of 1:2?

Can someone help me w/this problem? I have no idea how to do it. I tried setting it up like this but confused:

K = [c][d]^2/ [a][b]^2 .....

Thanks! n the answer is 9.97 * 10^8