Explanation

(b)
$The\ general\ term\ in\ the\ expansion\ of\ \left( x - \frac{1}{x} \right)^{18}is$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ T_{r + 1} = \ ^{18}C_{r}(x)^{18 - r}\left( - \frac{1}{x} \right)^{r}$
Here, $n = 18$
$\therefore$ the middle term is $T_{9 + 1},$ where $r = 9$
$\therefore\ \ \ \ \ \ \ \ T_{9 + 1} = \ ^{18}C_{9}({- 1)}^{9}x^{18 - 2r}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = -^{18}C_{9}x^{18 - 18} = -^{18}C_{9}$