a) Решим уравнение:
$$\frac{4}{9}y + \frac{1}{3}y = 6,3;$$
$$\frac{4}{9}y + \frac{1 \cdot 3}{3 \cdot 3}y = 6,3;$$
$$\frac{4}{9}y + \frac{3}{9}y = 6,3;$$
$$(\frac{4}{9} + \frac{3}{9})y = 6,3;$$
$$\frac{7}{9}y = 6,3;$$
$$y = 6,3 : \frac{7}{9};$$
$$y = 6,3 \cdot \frac{9}{7};$$
$$y = \frac{6,3 \cdot 9}{7};$$
$$y = \frac{0,9 \cdot 9}{1};$$
$$y = 8,1.$$
Ответ: $$y = 8,1$$