По теореме синусов:
$$\frac{BC}{sin(A)} = \frac{AC}{sin(B)}$$
$$AC = \frac{BC \cdot sin(B)}{sin(A)}$$
$$AC = \frac{7\sqrt{2} \cdot sin(30^{\circ})}{sin(135^{\circ})}$$
$$AC = \frac{7\sqrt{2} \cdot \frac{1}{2}}{\frac{\sqrt{2}}{2}}$$
$$AC = \frac{7\sqrt{2}}{2} \cdot \frac{2}{\sqrt{2}} = 7$$
Ответ: 7 см