(a)
Given equation can be rewritten as
$x(x - 2y) - 3(x - 2y) = 0$
$\Rightarrow \ \ \ (x - 3)(x - 2y) = 0$
$\Rightarrow \ \ \ x = 3,\ x = 2y$
$\Rightarrow \ \ x = 3,\ \ y = \frac{3}{2}$
$\therefore$ Centre of circle is $\left( 3,\frac{3}{2} \right)$