$(ii)$ $2 NO _2 \rightleftharpoons 2 NO + O _2 \quad K _2=$ (?)
$\begin{aligned} \text { (ii) } =-2 \text { (i) } \\ \therefore K_2 =\frac{1}{K_1^2} \cdot \frac{1}{\left(4 \times 10^{-3}\right)^2}=\frac{1 \times 10^{-6}}{16}=\frac{100 \times 10^4}{16} \\ =6.25 \times 10^4 \end{aligned}$