at room temperature dinitrogen tetroxide is in equilibrium with nitrogen dioxide. Deduce an expression relating Kp for decomposition reaction with "a" the degree of dissociation and "P" the total pressure.

I have tried it out using 100 as an initial number of moles of dinitrogen tetroxide. I got this answer:
[(2aP)^2]/(1-a^2) = Kp

Forget the other answer. COuld you check please?