Let $A = \begin{pmatrix} 2 & 3 \\ a & 0 \end{pmatrix}$, $a \in \mathbb{R}$, be written as $P + Q$, where $P$ is a symmetric matrix and $Q$ is a skew-symmetric matrix. If $\det(Q) = 9$, then the modulus of the sum of all possible values of $\det(P)$ is equal to: