(d)
Let $\alpha = \frac{3}{2} + \frac{7}{2}i$
$\beta = \frac{3}{2} - \frac{7}{2}i$
$\therefore\ \ \alpha + \beta = 3,\ \ \ \alpha\beta = \frac{9}{4} + \frac{49}{4} = \frac{29}{2}$
$\Rightarrow \ \frac{6}{a} = 3,\ \frac{b}{a} = \frac{29}{2}$
$\Rightarrow \ \ a = 2,\ \ b = 29$
$\Rightarrow \ \ \ a + b = 31$