9-4. Find all negative $a$ for which the equation

$$
\frac{8 \pi a-\arcsin (\sin x)+3 \arccos (\cos x)-a x}{3+\operatorname{tg}^{2} x}=0
$$

has exactly three solutions. In your answer, specify the sum of all found $a$ (if such $a$ do not exist, then specify 0; if the number $a$ is not an integer, then round it to the hundredths).
Solution. For $a=-1, a=-2/3, a=-4/5$. The sum is $-2.4(6)$.

Answer: $-2.47$.