Let \( A = \left\{ \theta \in [0, 2\pi] : 1 + 10\,Re\left( \frac{2\cos\theta + i\sin\theta}{\cos\theta - 3i\sin\theta} \right) = 0 \right\}. \) Then \( \displaystyle \sum_{\theta \in A} \theta^{2} \) is equal to:
\( 8\pi^{2} \)
\( \frac{27}{4}\,\pi^{2} \)
\( \frac{21}{4}\,\pi^{2} \)
\( 6\pi^{2} \)