9-3. Find all negative $a$ for which the equation

$$
\frac{6 \pi a - \arcsin (\sin x) + 2 \arccos (\cos x) - a x}{\tan^2 x + 4} = 0
$$

has exactly three solutions in the set $[\pi, +\infty)$. In your answer, provide the sum of all such $a$ (if no such $a$ exists, indicate 0; if the number $a$ is not an integer, round it to the nearest hundredth).
Solution. For $a=-1/3, a=-2/3, a=-3/5$. The sum is $-1.6$.

Answer: $-1.6$.