(b)
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$Rate = - \frac{1}{2}\frac{d\left\lbrack {NO}_{2} \right\rbrack}{dt}$
$\ = k_{1}\left\lbrack {NO}_{2} \right\rbrack^{2} - k_{2}\left\lbrack N_{2}O_{4} \right\rbrack$
$\therefore$ Rate of disappearance of ${NO}_{2}$
$ie,\ - \frac{d\left\lbrack {NO}_{2} \right\rbrack}{dt} = 2k_{1}\left\lbrack {NO}_{2} \right\rbrack^{2} - 2k_{2}\left\lbrack N_{2}O_{4} \right\rbrack$