(b)
Given, $x = \frac{\sin y}{\sin{(a + y)}}$
$\Rightarrow$ $\frac{dx}{dy} = \frac{\sin{(a + y)\cos y}\ - \sin{\cos{(a + y)}}}{\sin^{2}{(a + y)}}$
$= \frac{\sin{(a + y - y)}}{\sin^{2}{(a + y)}}$
$\Rightarrow \ \frac{dy}{dx} = \frac{\sin^{2}{(a + y)}}{\sin a}$