$$\frac{8.6}{4.8} = \frac{z+1}{2.4}$$
$$z+1 = \frac{8.6 \cdot 2.4}{4.8} = \frac{8.6}{2} = 4.3$$
$$z = 4.3 - 1 = 3.3$$
Ответ: z = 3.3