(b)
Since, $n$ is even, therefore $\left( \frac{n}{2} + 1 \right)$th term is the middle term.
$\therefore T_{\frac{n}{2} + 1} = \ ^{n}C_{\frac{n}{2}}\left( x^{2} \right)^{\frac{n}{2}}\left( \frac{1}{x} \right)^{\frac{n}{2}}$
$= 924x^{6}$ (given)
$\Rightarrow \ x^{\frac{n}{2}} = x^{6}\ \Rightarrow \ n = 12$