$$y - \frac{4}{7}y = 4,2$$
$$\frac{7}{7}y - \frac{4}{7}y = 4,2$$
$$\frac{3}{7}y = 4,2$$
$$y = 4,2 : \frac{3}{7}$$
$$y = 4,2 \cdot \frac{7}{3}$$
$$y = \frac{42}{10} \cdot \frac{7}{3}$$
$$y = \frac{14}{10} \cdot \frac{7}{1}$$
$$y = \frac{98}{10}$$
$$y = 9,8$$
Ответ: $$y = 9,8$$