(a)
$\because$ Eccentricity of ellipse$= \sqrt{1 - \frac{b^{2}}{a^{2}}} = \sqrt{1 - \frac{3}{4}} = \frac{1}{2}$
$\because$ Eccentricity of hyperbola$= 2$
$\Rightarrow \ \sqrt{1 + \frac{b^{2}}{64}}\ \Rightarrow \ \ 2$
$\Rightarrow \ 4 = 1 + \frac{b^{2}}{64}\ \ \Rightarrow 192 = b^{2}$