По теореме синусов:
$$\frac{BC}{\sin A} = \frac{AC}{\sin B}$$
$$AC = \frac{BC \cdot \sin B}{\sin A}$$
$$AC = \frac{4\sqrt{6} \cdot \sin 60°}{\sin 45°}$$
$$AC = \frac{4\sqrt{6} \cdot \frac{\sqrt{3}}{2}}{\frac{\sqrt{2}}{2}}$$
$$AC = \frac{4\sqrt{6} \cdot \sqrt{3}}{\sqrt{2}} = \frac{4\sqrt{18}}{\sqrt{2}} = 4\sqrt{9} = 4 \cdot 3 = 12$$
Ответ: 12