Решим уравнение:
$$ (x+10)^2 = (5-x)^2 $$
$$ x^2 + 20x + 100 = 25 - 10x + x^2 $$
$$ x^2 + 20x + 100 - x^2 + 10x - 25 = 0 $$
$$ 30x + 75 = 0 $$
$$ 30x = -75 $$
$$ x = -\frac{75}{30} $$
$$ x = -\frac{5}{2} $$
$$ x = -2.5 $$
Ответ: -2.5