8) \frac{21c^3}{13p^2} \cdot \frac{39p}{28c^2}

8) \frac{21c^3}{13p^2} \cdot \frac{39p}{28c^2} = \frac{21c^3 \cdot 39p}{13p^2 \cdot 28c^2} = \frac{819c^3p}{364p^2c^2} = \frac{9c}{4p}

Ответ: \frac{9c}{4p}