Verified answer

С * 1/z = m * 1000 / (M * 1/7 * V) | умножим на V

V * C * 1/z = m * 1000 / (M * 1/7) | поделим на С * 1/z

V = m * 1000 / (1/7 * M * C * 1/z)

V = 1000 * 7 * z * m / (M * C)

V = 7000 * z * m / (M * C)