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