(x + 10)² = (5 - x)²
x² + 20x + 100 = 25 - 10x + x²
20x + 100 = 25 - 10x
20x + 10x = 25 - 100
30x = -75
$$x = \frac{-75}{30} = \frac{-5 \cdot 15}{2 \cdot 15} = -\frac{5}{2} = -2.5$$
Ответ: -2.5