$NO _{2}( g )$$\xrightarrow \Delta$ $\begin{array}{*{20}{c}} {{N_2}{O_3}} \\ {\left( B \right)} \end{array}$
$N_2 O _{4}+ NO \stackrel{\Delta}{\longrightarrow}$$\begin{array}{*{20}{c}} {{N_2}{O_3}} \\ {Blue\,solid\,\left( C \right)} \end{array}$
$O.S.$ of nitrogen in $N _{2} O _{3}$ is +3
$N _{2} O _{3} 2 x +3(-2)=0$
$x=+3$