Explanation

(b)
Let $(r + 1)$th term be the coefficient of $x^{0}$ in the expansion of
$\left( x - \frac{1}{x} \right)^{6}$.
$\therefore T_{r + 1} = \ ^{6}C_{r}x^{6 - r}\left( - \frac{1}{x} \right)^{r}$
$= ( - 1)^{r}\ ^{6}C_{r}x^{6 - 2r}$
Since, this term is a constant term.
$\therefore 6 - 2r = 0 \Longrightarrow r = 3$
$\therefore T_{4} = {( - 1)}^{3}\ ^{6}C_{3} = - 20$