218 3) 5³ˣ + 3 ⋅ 5³ˣ⁻² = 140;

3) 5³ˣ + 3 ⋅ 5³ˣ⁻² = 140

$$5^{3x} + 3 \cdot \frac{5^{3x}}{5^2} = 140$$

$$5^{3x} + \frac{3}{25} \cdot 5^{3x} = 140$$

$$5^{3x} (1 + \frac{3}{25}) = 140$$

$$5^{3x} (\frac{28}{25}) = 140$$

$$5^{3x} = \frac{140 \cdot 25}{28} = 5 \cdot 25 = 125$$

$$5^{3x} = 5^3$$

$$3x = 3$$

$$x = 1$$

Ответ: 1