(b) 2.5 cm
Since △ABC ~ △QPR, corresponding sides are proportional:
$$\frac{BC}{PR} = \frac{AC}{QR}$$
$$\frac{5}{x} = \frac{6}{3}$$
$$x = \frac{5 \times 3}{6} = 2.5 \text{ cm}$$
Source: Chapter 6, Section 6.4 (Criteria for Similarity of Triangles)
The key step is identifying correct corresponding sides from the similarity statement △ABC ~ △QPR: A↔Q, B↔P, C↔R. So AC corresponds to QR, and BC corresponds to PR. Examiners expect students to read the correspondence carefully from the order of vertices, not guess from the figure.