e) -2 √0,49x⁴/0,81y² при х = 5/7, у = -2/9.

e) $$-2 \cdot \sqrt{\frac{0.49x^4}{0.81y^2}} = -2 \cdot \frac{\sqrt{0.49} \cdot \sqrt{x^4}}{\sqrt{0.81} \cdot \sqrt{y^2}} = -2 \cdot \frac{0.7 \cdot x^2}{0.9 \cdot |y|} = -2 \cdot \frac{7 \cdot x^2}{9 \cdot |y|} $$.

При $$x = \frac{5}{7}, y = -\frac{2}{9}$$, получим:

$$-2 \cdot \frac{7 \cdot (\frac{5}{7})^2}{9 \cdot |-\frac{2}{9}|} = -2 \cdot \frac{7 \cdot \frac{25}{49}}{9 \cdot \frac{2}{9}} = -2 \cdot \frac{\frac{25}{7}}{2} = -2 \cdot \frac{25}{7 \cdot 2} = -\frac{25}{7}$$.

Ответ: $$\frac{-25}{7}$$