(a)
For continuity, $\lim_{x \rightarrow 0}\ f(x) = k$
$\Rightarrow \ \ \ \lim_{x \rightarrow 0}\ \ \ \frac{\sin{3x}}{\sin x} = k\ \ \Rightarrow \ \underset{x \rightarrow 0}{lim}\ \ \frac{\sin{3x}}{3x}.\frac{3x}{\sin{3x}} = k$
$\Rightarrow \ \ \ 3 = k$