7) \frac{48ab}{17c^4} \cdot \frac{51bc^5}{40a^4}

7) \frac{48ab}{17c^4} \cdot \frac{51bc^5}{40a^4} = \frac{48ab \cdot 51bc^5}{17c^4 \cdot 40a^4} = \frac{2448ab^2c^5}{680a^4c^4} = \frac{36b^2c}{10a^3} = \frac{18b^2c}{5a^3}

Ответ: \frac{18b^2c}{5a^3}