You would need the number of moles of dinitrogen pentoxide present in the container, or at least a mass of the compound, in order to solve for its initial concentration.
Since no such information was given, I assume you have to express the rate law for this reaction in a more general way.
So, you know that dinitrogen pentoxide decomposes by a first-order reaction
This means that its rate law can be expressed like this
Let's say you have a mass of x grams of dinitrogen pentoxide given. Determine the number of moles present by using the compound's molar mass
This means that its initial concentration will be
The initial rate of decomposition will then be