$$b + a = \frac{1}{3} + 1 = \frac{1}{3} + \frac{3}{3} = \frac{4}{3}$$
$$\frac{1}{b+a} = \frac{1}{\frac{4}{3}} = \frac{3}{4}$$
$$\left(\frac{b}{a} - \frac{a}{b}\right) \cdot \frac{1}{b+a} = -\frac{8}{3} \cdot \frac{3}{4} = -\frac{8 \cdot 3}{3 \cdot 4} = -\frac{8}{4} = -2$$
Ответ: -2