3) $$14 \cdot ( -\frac{1}{7} \sqrt{15})^2 - \frac{1}{8} \cdot (2\sqrt{6})^2$$

$$\qquad 3) 14 \cdot ( -\frac{1}{7} \sqrt{15})^2 - \frac{1}{8} \cdot (2\sqrt{6})^2 = 14 \cdot \frac{1}{49} \cdot 15 - \frac{1}{8} \cdot 4 \cdot 6 = \frac{2 \cdot 15}{7} - \frac{24}{8} = \frac{30}{7} - 3 = \frac{30 - 21}{7} = \frac{9}{7} = 1\frac{2}{7}$$ Ответ: $$1\frac{2}{7}$$