The decomposition of $N_2O_5$ is given by the following equation: $2\; N_2O_5\; (g) \rightarrow 4 NO_2\;(g) + O_2\;(g)$ At an initial $[N_2O_5]_0 = 3.15\; M$, the initial rate of reaction is $5.45 \times 10^{-5} \;M/s$ and when $[N_2O_5]_0 = 0.78\; M$, the initial rate of reaction is $1.35 \times 10^{-5}\; M/s$. What is the rate constant of this reaction?