If $x^2+y^2=a$ and $xy=6b$, then which of the following is equivalent to $3a-2b ?$

Question

$\begin{array}{1 1} (a)\; (3x-3y)^2 \\ (b)\; (3x+3y)^2 \\ (c)\; (\sqrt{3}x-\sqrt{3}y)^2 \\ (d)\; (\sqrt{3}x+\sqrt{3}y)^2 \end{array}$