Подстановка условия
[src]
$$\operatorname{acos}{\left(2 a \right)}$$
$$\operatorname{acos}{\left(2 a \right)}$$
$$a = 3$$
$$\operatorname{acos}{\left(2 (3) \right)}$$
$$\operatorname{acos}{\left(2 \cdot 3 \right)}$$
$$\operatorname{acos}{\left(6 \right)}$$