given this reaction

N2O5 -> 2 N2O + 1/2 O2

all gases and given the initial pressure (P Initial) and total pressure (P total)at any time, relate the partial pressure of N2O5 (PP N2O5) to P total for any time during this reaction. Consider gas kinetics.

My teacher says the following is the answer.

(PP N2O5)= -2/3(P total)+5/3 (P initial)

My answer didn't look anything like this. Someone please suggest how you get from the top recation to this relation.