(b)
Since, $f(x)$ is the integral function of $\frac{2\sin{x - \sin{2x}}}{x^{3}}$, therefore by definition
$f^{'}(x) = \frac{2\sin{x - \sin{2x}}}{x^{3}}$
$\Rightarrow \ \ \ \ \lim_{x \rightarrow 0}\ f^{'}(x) = \lim_{x \rightarrow 0}\frac{2\sin{x - \sin{2x}}}{x^{3}}$
$= \lim_{x \rightarrow 0} = \frac{2\sin x}{x}.\frac{1 - \cos x}{x^{2}} = 1$