Answer
$-\frac{\Delta [N_{2}O_{4}]}{\Delta t}$
Work Step by Step
The rate of a reaction in terms of a reactant with stoichiometric coefficient 1 is equal to $-\frac{\Delta [R]}{\Delta t}$ where [R] is the concentration of the reactant. Here, R or the reactant is $N_{2}O_{4}$.
Hence the rate of reaction= $-\frac{\Delta [N_{2}O_{4}]}{\Delta t}$. Note that $\Delta [N_{2}O_{4}]$ is multiplied with -1 to make the rate of the reaction a positive quantity.
