Используем теорему синусов:
$$\frac{a}{sin A} = \frac{b}{sin B} = \frac{c}{sin C}$$
В нашем случае:
$$\frac{BC}{sin A} = \frac{AC}{sin B}$$
$$\frac{6\sqrt{2}}{sin 45°} = \frac{AC}{sin 30°}$$
$$sin 45° = \frac{\sqrt{2}}{2}$$
$$sin 30° = \frac{1}{2}$$
$$\frac{6\sqrt{2}}{\frac{\sqrt{2}}{2}} = \frac{AC}{\frac{1}{2}}$$
$$6\sqrt{2} * \frac{2}{\sqrt{2}} = AC * 2$$
$$12 = 2 * AC$$
$$AC = 6$$
Ответ: 6