а) $$(z - 6) \cdot \frac{3}{7} = 3$$
$$z - 6 = 3 : \frac{3}{7}$$ $$z - 6 = 3 \cdot \frac{7}{3}$$ $$z - 6 = \frac{3 \cdot 7}{3}$$ $$z - 6 = 7$$ $$z = 7 + 6$$ $$z = 13$$
Ответ: z = 13