For the equilibrium amounts. Why dont we divide by 2dm^3 to get concentration then substitute into Kc to get answer ?

Why is it not 1-a/2 and a for concentrations of N2o4 and no2 respectively

Oops, my bad!! Yes we should divide by 2 to get the concentration, this will result in the C to be the true answer (didn't see the 2dm3 volume part). So basically what you did is correct. However this question is pretty vague that unit of Kc is not defined, as one can decide the unit of volume, which will change the answer.

Thank you sm!