По теореме синусов:
$$\frac{BC}{sinA} = \frac{AC}{sinB}$$
$$AC = \frac{BC \cdot sinB}{sinA} = \frac{6\sqrt{2} \cdot sin30^{\circ}}{sin45^{\circ}} = \frac{6\sqrt{2} \cdot \frac{1}{2}}{\frac{\sqrt{2}}{2}} = \frac{6\sqrt{2} \cdot 1 \cdot 2}{2 \cdot \sqrt{2}} = 6$$
Ответ: 6