12. Given the quadratic curve $x^{2}-8 x \sin ^{2} \theta+4 y$ $+16 \sin ^{4} \theta+4=0$ has its focus on the line $x-y=3$, then the value of $\theta$ in the interval $\left[\frac{3 \pi}{2}, 2 \pi\right]$ is $\qquad$ $\bullet$
12. $\frac{11}{6} \pi$.